
I tried to draw different kinds of perspectives. I think the choice comes from what kind of a game you are thinking of.

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1. is traditional perspectiveless-blocks - background objects are separated by them being lighter than the foreground and maybe having lighter outlines? Mario is an example, of course.
2. Point and click, of course you have to make different character sizes. Not really relevant, for some reason I included that.
3. Beat-em-up, characters can move up and down slightly. Background objects should still stay off the walk-plane, and the walkable area should be clearly different (a road, a fence, something like that)
4. Kinda what the others are suggesting, I saw jazz 2 do this. Basically the character walks right in the middle of the ground, with the groundplane extending slightly above and below the walkable area. Jazz 2 had pretty lenient jumps, so it wasn't an issue. Notice that the walkable area again should be kinda separated from the background visually so the player doesn't get confused about where they are able to land.