What I would do in terms of the roster

Lobbers- 2 plants
Cabbage pult- leafy lobbing leaves lasting lashes on lingering lifeforms foreign to the lawn.  Not much to say here other than it can hit flying enemies.
Seed spitter- seed spitter has three distinct traits- lobbing, area limited multihit, and being a fruit. Many fruit plants have area limited multihit, like melon pult with a 3x3 area of effect, akee being able to hit multiple zombies in its own lane, ghost pepper (which are fruits apparently) have a 1x3 attack range before exploding, bombegranite, pepper pult, solar tomato, theres a lot of limited area multihit fruits (I want to call it that so it can fit the likes of aoe like melon pult and contained bouncing like akee, and if you want to you can consider starfruit and dragon bruit although those just have multiple projectiles). Seed spitter itself would spit a hard seed, allowing it to bounce to another zombie in the same lane. I think the pure tier 3 would be akee, but im not sure what the tier 2 would be. I could also see akee being the tier 2, with apple mortar being a tier 3.

Support- 5 plants
Tangle Kelp- A very versatile plant, it stuns on land and deals great damage in the water. I wish this was more reflected in plants like zapture though. I also wish more biomes had water to begin with, like at least sewers. Leaky pipes would be a fun obstacle to temporarily change up the battlefield to power up your amphibious plants.
Wall Nut- I like wall nut a lot, I really do. I just think he charges too quick. I think walls are important and im glad he costs more now, providing support without directly affecting zombies really makes the game a lot more enjoyable since you can do things that arent just obliterate everything on sight, and its actually strategy instead of just min-maxxing. Strategic depth is very important Thats really all I have to say about him. Maybe he could make kiwi beast, using something like phat beats, maybe incorporating lychee too if it would be a tier 3?
Blover- Since instant use plants are basically off the table, I think blover could be a staller. Periodically pushing back zombies, especially fliers, is a fun niche I think. It would combo well with other stalling tools like chill, and increase the potency of plants like spikeweed. I don't want to add plantern, at least as a tier 1, because I cant think of a mechanic for its evolutions. Ill mention it here- maybe if plantern is a tier 2 we could get disco plants like disco chomper or a disco ball citron, since plantern both has glass and makes light. I do think I want to come up with some sort of glass plant though, since magnifying grass exists. One thing I do wish though is maybe that blovers could push ambushes/drops forward, like the meteors in the volcanos, so they are less likely to hit plants or take up important backline areas, alongside other future airborne threats.
Umbrella Leaf- I never would have thought about a counter attack mechanic- its brilliant and fun and makes umbrella leaf fusions fun. I just think there isnt enough triggers for it. Blocking attacks is really cool. Honestly this is probably the number one plant that makes me want to argue for 5 seed slots, maybe 6- I just think support plants like umbrella leaf deserve to be taxing on your seed slots. Of course you can neglect to bring it and replace your plants- so its not a necessity ever, especially if we get something like hot potato to break up those saganaki mounds. Maybe in the sewers we could get zombie goldfish that rain from the sky, since people often flush zombie goldfish down the toilet. I think umbrella leaf could deflect them.
Grave Buster- Grave buster could deal significant damage over time to graves instead of just destroying them in a set time. This allows it to exist long enough to evolve which I think is fun. Also- its a vine, at least in pvz2, so I want to dual purpose this plant- planting it on a plant provides a thorny vine, akin to pumpkin, which can be evolved to create the vines from pvz2 (or at least some). I think a design like this helps players choose different plants, since you would only evolve one plant at a time- if they want the vines, they have to put plants in them that dont evolve with the same plants the vine does. This isnt even too limiting to the player since they can just wait for the cooldown, but it still provides thoughtful gameplay that might encourage players to switch up what they use from time to time, if they really want to. In a game about making choices matter I think a plant like this is engaging and thought provoking without requiring a challenge. Its also just nice to have ambiguous plants (like tangle kelp being plantable on land) to allow players to use familiar plants and unlocked merges in levels without the plant's main function.

Gonna cover some elemental plants here- gonna try to do one permanent attacker and one "instant use" plant
Electric- 2 plants
Zaptail- I still think they should be amphibious. They're reeds. Reeds grow in the water! Other than that I think its cool. Zapture is goofy though I already suggested a rework in another thread. 
E.M.P.each- I might have mentioned it in the robots/elements rework forum? E.M.Peach is a charge plant that unleashes a large burst of electric damage when a zombie approaches it. The more charge levels it has the bigger the area and the higher the damage. E.M.Peach serves to stun mechs using the electrical system, and is fully charged when planted. This hopefully emulates its ability from pvz2 while providing diverse usage for it and not making it an instant use. To avoid players emptying the charge and evolving just to get full charge on a higher tier plant, E.M.Peach and its evolutions can only evolve while fully charged. EMPeach (for short, I guess) can evolve into plants like citron.
Cold- 2 plants
Snowdrop- I want to get this plant in as a tier 1, since we know this game and heroes take place around the same time since both have the meteor Z plot point. I think a redesigned bellflower is in the files too, and the plant heroes as t4s. I have no idea what it would do without just copying gardendless. Im trying to think of it getting more powerful when chilled zombies are near, maybe creating a snowstorm. Maybe it harnesses power from chilled/frozen zombies and plants, creating snowstorms that get more intense the closer the nearest chilled zombie is. Think about it like the void spell from clash royale, which can hit everything in a wide area, but has less damage if its hitting more things. Likewise, snowdrop would create a weak but large snowstorm when the closest chilled zombie is far, but the closer it gets the more concentrated the snow is, eventually chilling by itself with the snowstorm. Higher forms would be able to target further zombies while still retaining strength relative to the closest chilled zombie. Im not sure how versatile this mechanic is, maybe this is just its own tier 2 and thats it, and some other t1 snow attacker would be added instead.
Iceberg Lettuce- It feels really weak honestly, especially with how many zombies there are. I think its fine because it encourages you to merge it but I really do think that if youre bringing an entire seed slot for an "instant" use staller that costs sun it should be able to hold at least one zombie permanently if replaced. I also feel like snow pea and cold slaw are really just not very unique. At least numbing cabbage and iceweed are unique since numbing cabbage has more range and damage, while iceweed can hit more zombies at once. I suggested in another thread that cabbage evolutions make bigger cabbages instead of just throwing more, which would allow the cabbage to deal splash damage similar to melon pult, or in this case, winter melon. Going back to iceberg lettuce though, its also currently the only elemental to only be 25% (or 1/4th) a part of its families (whereas gremonade and lightning reed are 50% or 1/2 of their tier 3 elemental families) which I find strange, although I assume they wanted to make it different since dangerine inherits splash soley from gremonade's extra area and not as a base effect of any of its t1s, so using winter greens instead of blockwood as the pure fusion for the cold t3 family just makes every elemental t3 do splash damage. Going back to the plant itself though it would be cool if it could freeze water like the ice monkey in btd6. I know this is like the 4th amphibious plant i'd be suggesting at this point but- cmon- if youre gonna remove lily pad at least let us freeze the water 
Poison- 2 plants
Poison ivy- I honestly a second groundcover plant like spikeweed would be interesting to see. It would most likely inflict a damage over time debuff too, so it wouldn't need to be paired with a wall to be most efficient. It could make other poisonous plants like shadow shroom, and also provide us with other pvzh plants like poison oak.
Lime bomb- I really hope this thing ends up making dragon bruit. Maybe as a tier 3 between like, seed spitter, threepeater, and whatever shadow tier 1 we get if we do get one? I know they want to keep tier 3s down to consisting of 3 tier 1s at most so they can have some semblance of identity and they cant just roll a random 4 plants and make weird garbage that doesnt work, which I applaud. Maybe remove seed spitter at that point or just use it being a tier 3 as an excuse to have 3 heads without threepeater. Alongside dragon bruit I cant wait for kill lime pie, I think everyone has been waiting for it. Definite posterboy of promising design. My only issue is that as a trap plant its just so dang similar to iceberg lettuce and potato mine- its really just mimics them in its pure fusions and uh... has nothing else except dangerine so its really hard to say anything, especially when potato mine really doesnt have many evolutions to differentiate itself from the other two. I think lime bomb could also be used to make lava guava, and maybe be more similar to it by leaving an acid puddle on defeat, that way it can be more unique and appealing. I also came up with the name "Lemonition" which is kinda similar to gremonade, maybe that could be a different tier 2 for lime bomb.
Fire- 2 plants
Cinderwood- From what we know, cinderwood isnt going to focus entirely on boosting peas but moreso on its special abilities from pvz2, those being its incredible health, melee damage, and jalapeno explosion on death. While I feel like a wall with a mild melee chip damage is just a weird endurian/lychee it is interesting and unique. I just think as a wall it should at least have the creative freedom to focus on its tree traits, and not just the fire traits. Im glad its doing more than just boosting pure peas which right now is just one tier 1 and you cant even evolve it with other plants. I did make more t1 peas to make torchwood more interesting but it may not even need that. 
Hot Potato- When you level up hot potato in pvz2 it gains the ability to break more obstacle types. I think, regardless of level, it should be able to break more obstacles, notably saganaki blasts. Allowing it to "live" longer on higher hp obstacles allows it to be evolved. Giving it a proper lifespan and not making it an instant use is perfect for what pvz3e is aiming for