Erm... well, I'm not a RPG expert, but I do enjoy Morrowind. If you haven't played it already, that is :P
Also, Ultima Underworld is pretty boss.
Seconded on both counts. I'd also add if you like old school and where you liked the BG series, you'd probably enjoy the Ultima VII games too. They are awesome. In their day, they were well ahead of their time.
If you want something newer and think you'd enjoy trying out a big huge open world to explore as you wish, doing quests for various factions as however pleases you and getting around to the main quest of the game when and if you feel like it, Morrowind is amazing. I had so much fun playing that game that it is one of the very, very few I've ever played that I know I want to replay again. I did a lot of quests and began the main story quest but I never did finish it and that was with over 100 hours of very fun playtime into the game. I became engrossed in just exploring and taking over the world as I went. My character rose to become very powerful over time and I wound up raiding the capital city and stealing the contents of its treasury for some really sweet armor and weapons just because it was fun trying to see if i could. Well, I did and I was well geared after that!
For someone who enjoys RPGs it would probably be pretty hard to go wrong with any of these, although there are a great many other classics you could also consider and perhaps other people will make mention of them here. I think that's enough rambling out of me for now. ;-)