You can just go get Unity or UE right now. With UE you can make a $1million before you need to pay a royalty and the tooling is substantially better than any of the tools id released back in the day. (And fwiw I think it’s a crying shame id tech engines are no longer open sourced too!)
Making a brand new game with an existing engine is not “build on top of one another like mods do”, though. Games engines do not include the game logic of the games.
You can just go get Unity or UE right now. With UE you can make a $1million before you need to pay a royalty and the tooling is substantially better than any of the tools id released back in the day. (And fwiw I think it’s a crying shame id tech engines are no longer open sourced too!)
Making a brand new game with an existing engine is not “build on top of one another like mods do”, though. Games engines do not include the game logic of the games.