Well if you are looking for the same playstyle as in first person open world RPG but your looking for something more Sci-FI I would go with bethesda's Fallout series. Almost identical in game play to the Elder Scrolls series but set in an apocolyptic wasteland so many hundred years after atomic bombs were dropped, it's awesome really, just as fun as the elder scroll series. Same gameplay as in you have skill sets your SPECIAL (strength, perception, endurance, charisma, intelligence, agility, and luck) you assign points to them like in oblivion and you get perks with level up that affect energy weapons, blunt, guns, explosives, medical, repair, and stuff like that. Really fun game, give it a shot. There is Fallout 3, and Fallout new vegas, and I haven't played any of the others but those two were awesome, especially with the DLC.
The Mass Effect series is pretty friggin good as well, a very vast universe... if you ever played dragon age it's pretty much the same gameplay wise with a few tweaks here and there same as fallout is to elder scrolls. Mass effect and Dragon Age are both bioware so that's fun. Very Sci-fi.
If you are looking for old school final fantasy style RPG, I would highly reccomend lost odyssey, it's four discs on xbox360 and plays exactly like an old FF game (not the newer) and Blue Dragon is alright, a little childish and corny at times but still fun.
ALSO, if you are looking for something more modern, not extremely sci fi as in space exploration and all that, check out Deus-Ex human revolution, that game is really fun at times too
Hope this helps, if you give me a little more info on what your looking for I might be able to help a little more.