Instead of making an engine (ie program that helps you make games), start making games, while you keep a "library" of useful generic code as much separate from the game as you can.

It's a double edged sword in this way. You're making programs that users can play, you get a as small and lean as possible "engine" (the library helps in making games, so it can be considered an engine), which in addition is also perfectly tailored for your kind of games.

Last but not least, it also solves the problem of deciding what functionality should be in the engine (which you can only find out by making loads of games, and understanding what the common factor in them is).