I still haven't managed to get Gothic III to install. This is the third computer since I bought it. And it came with Dungeon Lords, which installed fine.
At any rate, Konjad pretty much listed a whose-who on newer games. You might also try Wizardry 8. Not entirely like Oblivion, but IMO one of the best RPGs around that era. It's also older, and out of print.
Both Arena and Daggerfall, the precursors to Morrowing and Oblivion, are both freeware now. Google them. They might need DOSBox to play (Arena I'm almost 100% positive it will).
Might and Magic VI and VII were the last of the old Might and Magics worth playing. They're easy to find on Amazon, and VI is actually up on GoG.com along with 1-5. So you get a number of good old games. All but 6 will need DOSBox, and 6 is touchy with WinXP. It'll run though.
Going back further into the DOS days can get you a lot of really good games, but they're often really hard to find too.