Suggestions for game development that wont be a waste of time?

I want to mess around with Unreal Engine or possibly Unity. I was thinking of doing my own Real Time Strategy game (like Red Alert / Command and Conquer). But instead of starting something that big, perhaps there is something smaller I can build that might even show some returns. For example, it might be better to try and do some sort of short animation because then I can do short videos for cash. Or perhaps I can do a one-level game with the option to expand?

I'm not asking for ideas for games, but rather if there is a profitable way to learn Unreal Engine by actually building something useful.

Alternatively, I guess, I could try and get work for some gaming company. But I'd prefer just to do this for fun on the side.