Making your game general

ITT we discuss how to collaboratively design our games.

by keeping a game single player and 2D, you are spending less time on graphics and engine work

Nope, by taking a FPS engine(Unreal 4) and adding some RPG elements + minimal maps you can have a decent Action-RPG in a month. Without non-combat content is actually far easier to design a RPG around the FPS concept.