Which D&D edition (1-5 and coming soon 5.5) would you use for your Ravenloft fantasy video game? Power checks and magic being affected differently is a must as a part of Ravenloft same with animal companions. Mephisto above made a good point about forlorn. The original forlorn setting is open ended with players going where ever they like. Not positive, but I think nightmare lands and Masque of the red death (colonial era ravenloft) might be too.
So would Islands of terror be an expansion pack lol? Although there is the grand conjunction that changed everything some dm's can claim time distortion in the mists to allow players to bounce from adv to adv or pre conjuction domain to post grand conjunction domain. Just remember though time games are tricky (not sure how I feel about that personally). Castle Tristeniora is all about time bouncing between 3 incarnations of the castle with during 3 time periods. Enough rambling from me

Till the Goblyns devour....