as said above, that innocent question is one of the most difficult to handle.
to keep savefile compatibility, all your updates have to be incrementel - only adding things, never removing them, and always applied in sequence (no way to have optional updates unless you really know how to reprogram the engine).
even then, changes to the plugin manager are absolutely forbidden - as soon as you add or remove a plugin all players will be required to start new games.
If you can handle those restrictions (which will take some experience and won't happen on your first game), then additionally I suggest adding a plugin that let's you specify a savefile path, because most players will have problems extracting the saves from the browser data.
…I think I might need to switch from standard ATK/DEF up/down buffs to parameter altering states. I just had an actor whose usual attack against an enemy does ~25 do a buffed attack against a debuffed enemy for ~600…
Zombie status is now complete. I didn't have to do much in the way of changing how healing is applied to undead targets, was just a simple 3 line code. I thought I had to overhaul Game_Action lol. Everything else was doable by events. So.. Zombie: You turn undead. Your element changes to darkness. You're weak to light, you're strong to physical attacks and healing hurts you and revival items will kill you.
My Overworld might be a mix of inspiration from 2D Final Fantasy games, Breath of Fire, Battlechasers: Nightwar, Super Mario World, and Pillars of Eternity. If I can make it more in-depth than just walking from point A to point B, that might do a lot for the gameplay experience. Hmm...