Depends on what you're looking for exactly. If you want another fantasy RPG with tons of decisions and rewarding exploration there's Gothic and its successor series, Risen. If you want an immersive first person RPG there's the Stalker games (obviously those are sci-fi / horror, though). The closest thing you can get to Skyrim in terms of sheer scale would probably be The Witcher 3.

wut ?

https://www.gog.com/forum/general/games_similar_to_the_elder_scrolls_or_world_of_warcraft