-I want a game with a huge (even bigger than Oblivion's) game world
-It should be a non-linear open world where you can go anywhere from the start
-It should have a 1st person view
To be quite frank, you're going to be hard-pressed to find games that fit
all three of those criteria, especially the 1st-person perspective. If you've listed these requirements in relative order of importance, I would say go with Arcanum and the first two Fallout games: they have more quests and more explorable square-mileage than almost any other RPGs, easily enough to get lost in for months. If you're more inclined towards the last two requirements, you might look at Vampire: The Masquerade - Bloodlines, which is a great 1st- (and 3rd-) person RPG with a mostly non-linear plot; however, the maps will seem rather cramped. I'm not sure if I actually know any RPGs with worlds bigger than Oblivion's (except Daggerfall and maybe Morrowind) and certainly not any with a 1st-person view... Mount & Blade, as mentioned, does come to mind, but it's not really so much an RPG (with quests and a plot) as it is a medieval warfare simulator.
Basically, what you're really looking for is Skyrim.