To be fair, I believe balance is something you do for a mechanic, and a mechanic is something you implement into a game. Balance, to be fair however, can be a pain sometimes. Especially with a large cast/enemies/mechanics.
That said, I believe the hardest thing to implement into a game is any form of diverse lighting. Now, keep in mind, this is coming from someone that very much still needs to learn a lot (an amateur), but when I have to adjust lighting or if some character has to hold a lamp or flashlight, I have a hard time.
Additionally, any time I have to do some eventing, and there's a lot of variables involved that changes up said cutscenes. It's still something I have trouble with.
I'm still having a hard time implementing a CCG type function >_< I almost had it in VXA, but then my puter gave up the ghost and took the backups with it. (lesson learned: keep a hard copy of your code!) I hadn't used JS in a decade so I'm rusty AF and banging my head on my desk trying to get it working in MV.
…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...