I'd have a ton if I listed it all, but I'll hit on the high and low points:
1-The free roam zones are both one of my favorite things in BfN, but also end up kind of pointless after you 100% them. I love the quests, hunting down the chests and gnomes, solving the puzzles, and the general themes of these places but they need more depth and reasons to return consistently to make them worth the time and effort that went into them. I touched on this in my own ideas for a sequel; always have a reason to return to these zones and the game replay value would increase exponentially.
2-The Grandmaster ranks are more obnoxious than anything else. They provide almost zero benefit and just feel like they were added to keep people on a treadmill. If you're going to add to the treadmill, I'd like to see more rewards to make it worth it.
3-I largely enjoy the new characters, except for the swarm characters who I hope do not return in a new shooter. I've been pondering variants for them for a while now, and we've mentioned that before. I do wish we had gotten at least the Iceberg Lettuce, but I really wanted that Berry Brigade.
4-To your point 2, @Fork_x_Spoon I feel that GW2 had this issue with turrets too; too many turret characters. In GW1 we had the Peashooter's Gatling and the Sunflower's sun beam. In GW2 they added the Pirate's cannon rodeo and after taking the drones away from the engineer, they gave him the big bolt blaster. I'd like to see less AoE's and turret modes in a new game.
5-I honestly love all 3 games' visual style and they all have varying degrees of realism over different aspects. I generally don't like gritty realistic games either. This is why the PvZ shooters are my favorites and close behind them are Paladins and TF2 for my multiplayer shooters. BfN generally looks great, and some of the costumes are excellent.
6-The HUD is both better and worse than the prior games. For example, I love having a visual indicator about your overheat status and being able to visually tell how much of a % of your magazine you have left with a similar ring on the ammo indicator, but as @stukapooka has also pointed out, I wish the name of my main weapon at the least, appeared on the HUD.
7-Lobbies in the next game need to be better balanced than the current 3. Rank plates are not automatically better players but it is an indicator of general awareness of how the game works. If the next game has crossplay across platforms (and it should, it needs it), that would help a lot. Jumping into the portal in GW2 and being on a team with 3 other max ranks while the enemy team has none is not ideal.
8-Giddy Park isn't bad and I love the way they change appearance based on the current prize map, but it's just not as fun or interesting as the Backyard Battleground. The zombie base is too 'clean' (it should be much like it is in GW2 or Zomboss Estate from GW1), you can be spawn camped too easily with the current design, the AI are too limited (no bosses, no turrets, only wildflowers and TV Heads is a shame), and it's all divided up so it comes very disconnected.
9-Wildflowers and TV Heads (the AIs especially), are super annoying with their attacks that throw you around and ruin the normal melee focus of the Mass AI like in GW and GW2, making some bots/pots that are already limited due to range, like the Bonk Choy and Boxer Bot, even less useful than they already were. They should be removed for the next game and more interesting but generally melee-focused AI mobs should be added (ex: Pirate zombies of various types).
10-The economy is a major problem. One can argue about the packs in GW2 where you can buy the legendary characters being bad (and I'm not a fan of these to be sure), but the way outfits cost a ton of rainbow stars and they only trickle in unless you buy them (as I did), coins feeling like no reward after you have the reward-o-tron emptied, and regional currencies becoming pointless, all add up to a mess that made a lot of people unhappy, myself included, even if I put up with it.
11-The idea of monthly prize maps where you can get free stuff just for playing are a great idea, even if they weren't implemented well. They may want to use a more generic map in future and remove certain things (like poop emotes), so they can make them more easily and not repeat them like we have now, but the idea of 'play and get stuff' with no real restrictions is a good one.
12-Being able to save costumes and upgrades as separate builds is one of my favorite things in BfN. It was something I asked for in GW2 for years. 3 per character isn't enough, but it's still a great start. I love being able to just grab a pre-saved build and jump into a game quickly like that.
13-Ops are a major letdown in BfN. The lack of themed enemies (back up dancers for the disco zombie as one example), the lackluster special wave variety, and no evac stage really made these a disappointment.
14-I actually like the timer system for summonables so you can't spam nothing but coffin zombies like you see in GW1 and 2. But, it would be nice to have more variety of summons and for defense to have at least 2 that can always be summoned, not just the one per side like we have now. This would alter the coin expenditure we have in GW1 and 2, whereas we always get something for coins, so I wouldn't mind going back to a system like that, or one where you purchase them directly as I posted elsewhere (ex: a browncoat costs 100 coins, a coffin zombie costs 1000), or keeping and expanding this summonable system.
15-Losing mixed mode was a big bummer. I understand why it was taken away, but it still stinks. With crossplay and being able to combine the 3 player pools (PC, PS, XBox), we'd be able to get that back in a new game.
16-Battle Arena is a great idea that had so many issues with implementation that it never reached its potential. I'd like to never worry about ranked modes if it's brought back, and just have it as a fun and exciting game mode though.
There's likely more, but I've already had so many here that it's enough for now.
