I think this is the definition of side projects.

Like, if you stay focused, is it even really a side project?

Which is why my 2d top down sprite-based rpg now has a 3d procedural animation engine, a procedural 3d character generator with automagic rigging, a population simulator that would put Europa Universalis to shame if I ever get around to finishing it (ha!) a pixel art editor, a 2d procedural animation engine using active ragdolls.........

You might wonder why a 2d game needs 3d procedural animation, well...

The scope creeps in mysterious ways