Space War 2K: one engine, three games — in the year 2000
My inspiration at the time was Half-Life — specifically, how one engine could run completely different games as mods. So I went deep.
I first built a game about little ships fighting in space. Then I realised that the same engine could run top-down tank battles in the spirit of the first GTA. After that I made a Worms-like artillery game on it too.
The trick was that nothing in the executable changed. Each game was a directory with its own resources plus a config with the rules; one .exe ran all of them. It is the same idea I would use again years later at MadSword, where one engine shipped 100+ games.

Three games on one engine: the space shooter, the Worms-like game and the top-down game.
The main menu had a video. I rendered a sequence of frames in 3ds Max and wrote my own format that stored only the pixels that changed between frames — a simple delta codec — and played it back in Pascal, which had no notion of video at all. Pascal didn’t really do images either, so the BMP loader was mine too.