Game ideas aren’t necessarily relevant to whether it’s 2D or 3D.
But yes, 3D graphics are inherently harder, because what could be as simple as a bunch of sprites in 2D must necessarily be split into textures, materials, shaders and models.
This is why I pretty much am stuck working on my own 3D game framework, without making any real games with it. I just don’t have the time nor skill to work on all the assets without getting someone else on board.
There’s also the asset flip option. It’s possible to make game with third-party assets look good, but it can take a lot of editing.
EDIT: Oh, and there’s also the cheap 3D look. Check out something like Polly the Frog. There’s many games like it, but essentially they don’t even bother with shading and materials, because basically everything has only diffuse textures. But it’s a very specific vibe, and it won’t fit everything.