Adventure mode needs to change
The adventure mode right now is undeniably flawed. Over a hundred randomly generated levels, no special objectives, and it doesn't feel like much of an adventure at all when it doesn't unlock you anything significant past level 30. I think with some changes like these, it can be much better.
PLANT UNLOCKS First, and most importantly, the plants. You unlock all the plants early on and have nothing to look forward to after that. Even more egregious is that you don't unlock A SINGLE SEED that helps towards collecting evolutions. Once you unlock a plant, you're never getting another seed for it from here. Not only does this make it much harder to get fusions, especially as a new player, it goes against the entire point of the game. What's the point of calling your game PvZ Evolved if it takes days to unlock your first non-tutorial evolution?
Now, in this idea that won't be a problem anymore. The first world, your house, should unlock you a lot of tier 1 plants. Not only that, you should unlock at LEAST 50 seeds each for each of them. This may sound like a lot, but it will only get you about four tier 2 plants, which considering the whole point of the game is the evolution mechanic, it's surprising this wasn't in place already. Also, when you unlock a plant, you should get it's respective tier 2 as well. This is to make sure no matter what there is some fusing to be had, and to help make the Zen Garden less tedious.
LEVEL DESIGN The way it is now, the levels in this game are all the same. A randomly generated assortment of sod, or water tiles, maybe some lightning strikes if your lucky. I'm sorry to say Popcap, but adding in a bowler zombie every once in a while isn't enough. There really needs to be some creative, fun puzzle levels in this game. The possibilities are endless with the merging mechanic. Bring back the obvious ones, endangered plants, don't let zombies trample the flowers, mold colonies.
But let's try some new ones, too. Maybe a few levels where zombies merge together, two basics touch and become a conehead, two coneheads touch and become a buckethead. Maybe make a level that restricts the number of fusions you can do. Just SOMETHING other than EVERY OTHER LEVEL. (this is completely ignoring how flawed the wave structure is but im sure youve already heard enough about that)
WORLD STRUCTURE The first world is good as it is. Just getting the hang of the game for 20 levels. Should be a couple minigames thrown in there. The great thing is you really don't need a lot of puzzle/minigame levels, the fusion mechanic is a gimmick itself, and since in this idea of the game we unlock a lot more tier 2s easier, you won't get bored trying out different plant combos. Then, we move on to the campgrounds. Just two plants here, but we focus on them and spend these levels learning them and their fusions. Zaptail is the counter to the zombies here, and tangle kelp is an obvious choice for the water tiles. We'll make sure to actually use these plants in different ways, like endangered plant levels or by using zombies that counter them and make you think of a way around it.
The MAIN thing I want here, is you unlock a LOT of seeds for these two. At LEAST 100 each. Let us unlock evolutions in the game LITERALLY CALLED PvZ EVOLVED! It really baffles me how stingy the game is with these things. The main points here, use the zombies and minigames to properly teach the player about these plants over the course of around 20-30 levels.
TLDR: Make the worlds better teach the player how to use plants and counters with puzzle levels and actually letting them get a bunch of tier 2s. Each world should be around 25 levels.
I really think this game has a lot of potential but you've gotta listen to the fans. All I've said here is stuff I hear from people across every corner of the internet. Hell, even my friends and parents have some of these complaints, and THEY'RE the casual audience this game is trying so hard to please. You've got the opportunity to completely reinvigorate the PvZ IP, but you can't remove what made the last 2 so great if you really want success.