I can’t think of new suggestions, so I’ll comment upon the suggestions of others. I’m a fan of both Skyrim and Fallout 3.
I picked up Kingdoms of Amalur: Reckoning cheap a while ago. Big game, fairly linear (monsters acted as zone blocks), bad aesthetic. Suggestion: pass. There are heaps of areas to explore, though, and a ton of side quests.
I got into Witcher 2 recently-- this was my second pass at the game, the tutorial put me off the first time around. A medium-sized game masquerading as a big one, linear with meaningful choices, story-heavy (and cut scene heavy, but luckily mostly skippable), fun combat, good looking (although I got sick of seeing the hero’s back). There’s plenty of exploration to be had. However, everything you find is there for a quest. So if you stumble over some interesting ruins or a cave, you know you’re either coming back for a quest or about to trigger one. Suggestion: play, and if the tutorial tells you to play on Easy, take its advice. You can always turn it up later. Also, yes, there are mods, but they don’t look interesting-- the usual tweaks, cheats, and nudes.
Borderlands I/II, Torchlight I/II, and Far Cry 3 are all fine games, IMO, and listed in order of preference (I “paused” FC3 about halfway through, thinking I’d pick it up again; that was almost six months ago now). They aren’t RPGs the way I think of them, though, but shooters with level systems or Diablo-likes. However, they all have plenty of side exploration and room to do it in. Actually, with FC3, if you go off the path too often I think you’d be over-level for the main quest-- not that you couldn’t complete it, just that you’d blow through it.
Of the Dragon Age games, I played Origins, which is supposed to be the best of the series. I got sick of it. The NPCs remind me of KoA:R-- similar looking, standing around like they have roots, with just a barge load of stupid flavor text inside them that they’ll spew on you if you get too close. Combat was interesting, but I also found it frustrating and quit at a spike. I didn’t try getting over the spike for long. I don’t remember a lot of exploration. Actually, I think the game had a timer of sorts, where an invasion or spreading map stain was intended to hurry you along or at least keep the player from circling back.
The Mass Effect series is just plain cool. Linear with meaningful choices-- here decisions from the first game carry over into the second, lots of blah blah, fun combat with a small squad. The player class system was varied enough to tempt me into a replay. The only major minuses were stupid mini-games (driving in the first, mineral exploration in the second, haven’t played the third but I assume they added a bad mini-game for tradition’s sake). So there was exploration, they just wrapped it in annoyance. I can remember finding a few interesting spots here and there.