Game ideas: Medieval GTA
From GDWiki
How about a free roaming, open city game like GTA, but set in an alternate time period, like medieval times or ancient china? You could do missions for local citizens like a baker or a blacksmith. I think this could be an interesting game concept.
You can see how it's done in the Gothic, Gothic 2 and coming Gothic 3 game. So, I think mixing GTA + Gothic is really great idea!
That was pretty much done by Elder Scroll games (which were before GTA came to existance) Oblivion gives you ability to steal horses even. Grand Theft Ass!

