When I was a kid I loved this obscure multiplayer game engine called BYOND. In fact it's so obscure that even mentioning it provides several bits of fingerprinting. It technically still exists today, but it's been on life support for 15 years. We should make something like that again.

Besides the game engine, it provided central identity (optional - you could allow players to sign in as Guest), a website to browse games and servers, a forum to discuss games and programming, and an IDE with a built-in sprite editor (it was 2D), map editor and object browser.