Honestly, there are so many to name in my case that any list I would put up would be woefully incomplete.
So...I'm going to put one here's that's VERY out of the box.
Someday I will TOTALLY finish making that really big RPG I started working on like 5 years ago (and have had an on-off relationship with making since then)
I've made 5 out of the 8 planned dungeons, I have plenty of the later cutscenes done...But I keep getting discouraged with two giant bugbears.
1) Making dungeons is the part of RPG design I enjoy least. And for this game, every single dungeon revolves around a central puzzle style or gimmick that makes designing these so much harder than normal.
Like...The dungeon I'm supposed to be working on next is centered on leaving behind party members temporarily. I have no idea how the heck I'm going do design a dungeon like that, that feels properly satisfying while also preventing the player from trapping themselves in it.
2) The combat system is absurdly complex, with dozens of skills and modifiers to skills that the player can swap in and out as they please. And I love that about it. But it means that there's a certain expectation that every battle will have some nifty tricks or strategies to use, which makes designing enemies require a TON of thinking.
See, this is why I prefer making smaller RPGs for contests with tight deadlines! It prevents me from overscoping!
Ugh...Mods as my witness, I WILL someday finish making that game.