We don't know for certain Alduin won't have much personality, I mean, we haven't seen anything to indicate that he'll have any personality, but we haven't seen anything about him at all beyond knowing that he is behind the events of Skyrim. However, I suspect he will turn out to be one of those villains that has no real personality and essentially feels more like a device that exists to provide an antagonist for the story rather than an actual clearly defined character. Of course, we don't know this for certain, but "Dragon god that wants to destroy the world" just doesn't sound like the kind of antagonist that I'd expect to have a unique personality. But I could certainly see Bethesda adding in some sort of secondary humanoid antagonist serving Alduin, probably some sort of cult as it would be pretty hard to justify someone working for an entity that wants to destroy the world otherwise. Alternately, it could just be people with plans that you happen to be a threat to, and they either don't understand the threat Alduin poses, or think they have their own way to deal with it, after they've accomplished their current goals, but we'll see, right now, there's a lot about the game that's still up to speculation.
Well the users of Thu'um aren't nearly as powerful at it as the dragonborn or dragons. So you might find a evil Thu'um user but he won't be another dragonborn. As for you not being the subject of Oblivion, yes you were. You did all the work, Mehrunes Dagon was in no way going to be banished by you, a lowly mortal. You were instrumental in the closing of Oblivion and without you, kvatch would've still been overrun by daedra, Martin would've been killed, then Mehrunes Dagon would've destroyed the world. I mean, think about it. You closed the great gate in front of Bruma, something you and only you could do and so on. I didn't realize people saw that Martin turned into the Avatar of Akatosh and defeated Mehrunes Dagon as a affront to their heroism, I just assumed everyone saw it that Mehrunes really wasn't a boss. It never bothered me because I knew I was the true hero of the story and the only thing Martin did was help get the amulet of kings back and then sacrificed himself to banish Mehrunes Dagon. You don't have to defeat everyone to be the hero or focus of the story.
While it's true that the player did most of the work in Oblivion, I still feel that the game treats Martin as the real hero. Not because of the fact that the player isn't the one who defeats Mehrunes Dagon in and of itself, though that is kind of a letdown because the boss we get in his place was pretty underwhelming and the solution for defeating Mehrunes Dagon essentially amounted to a deus ex machina. It's more because, at the end of the day, Martin is the one who gets the real credit. I mean, Martin is the one people talk about as the chosen one who is needed to save the world. It's always "You need to save Martin!" or what not. Now, that alone doesn't make Martin the hero, as there are many stories where a secondary character plays a necessary part to accomplishing whatever needs to be done in the story, but the real hero is still the protagonist, but what really gets me is that at the end of the day, Martin is really the one who gets the credit. It seems pretty clear that Bethesda meant for Martin to be the real hero, despite the player being the one who does everything, and really, I think that's the problem with such a method of storytelling in games. In other mediums, you can have the protagonist be someone who isn't the one who does everything as long as that character is there to see it all. But in games, it will get pretty boring if you just stand aside and watch while oher people do what matters, so unless it's a game where you control more than one character, the main character pretty much needs to be the one who does the most important things in the story. And really, that's my problem with Oblivion's main quest, the story would have us think that Martin is the real hero, but we're always the ones doing the work, and at the end, it just feels less satisfying because you feel like you're not given credit for you're work.