I've been playing ES3: Morrowind for a bit via OpenMW, an open source reimplementation of the Morrowind game engine. It requires a copy of the purchased game itself, and uses the retail data to reconstruct the game. I'm confused about legality because you need to still buy the game itself, and it's otherwise nothing Bethesda worked on.
Given that emulators and engine reimplementations don't provide any IP material, I was thinking perhaps instead of a remake of CC or something, glance at the resources and what not and try to code an engine that reuses the files of a retail CC copy, possibly with or without some graphic enhancements, but definitely with things like mods or something, so you needn't even touch the original game files.
