Yeah, it's not an expansion at all. An expansion expands the original game. It doesn't mean it's completely different except the engine is the same. By your reasoning, Mass Effect 2 would be an expansion of ME1. The Elder Scrolls Oblivion would be an expansion of morrowind. Heck, since it's all the same engine, Fallout would be an expansion of the Elder Scrolls.
ok its not an expansion
im say it like street fighter 4
Super street fighter 4 is street fighter 4 but with a new coat of paint
it fallout 3 with a new coat of paint
instead grey it now yellow
im not saying its going to be bad
but this is what Obsidian do they take other people game and improve them and they have said that them selfs
yes it not expansion only cos it would makesmore money a stand alone game
but it could of been done as a DLC