A bunch of random suggestions
Screen door zombies with helmets - adds more difficulty and pressure to an easy to counter zombie.
More mini games - alot of scrapped minigames like unsodded would be interesting.
Maybe coffee been could have bonus functionality by speeding up plants for a short time?
Online vs mode - pretty self explanatory. Would be fun especially if you dont have anyone to play with.
Slightly higher difficulty or hardmode adventure - for the players who want a challenge because the base game can be too easy most the times.
Maybe have the female zombies from pvz 3? - adding some variety
The old buckethead and conehead designs where they have professions could replace the regular with a 1/5 chance to yet again add variety.
More vs mode stages? - maybe the pool, night, and roof? Obviously balancing would be made to compensate.
Maybe trashcan zombies could spawn in adventure as well as vs and survival/endless
Not necessary but would be cool if Brickhead came to the remaster
Maybe even a night roof, where zomboss could take place at 6-10 instead of 5-10. Its an area that fits in the base game and could maybe have a new mechanic??