Environments

I tend to use a lot of parallax layers to create depth in my environments. Everything else - shapes, colors, layouts - can be relatively simple.

Dimetric rooms

In a true isometric projection, all angles at an intersection of 3 orthogonal lines are equal.
In pixel art, the top of the tile is usually "squished" more than the sides.

Top-down assets

These can match surprisingly well with side-scroller props; a slight adjustment at the bottom of the asset makes it fit right in.

Props

These are background/foreground decorative objects. They are rarely animated and often reused,
but combining them in clever ways avoids repetitiveness.