I don't like rivals from the beginning. That just seems too scripted in some sense, unless something is done with the backstory to really tie this in. Otherwise, why on earth would anybody really *care* about being your rival?
I wouldn't mind the existence of rivals developing with the story though. So, you can have an opposing person/group also having their rise to power at the same time you are, but they're just on another side. The problem is that you don't want to kill them, but you don't want the conflict to be scripted too much where kills are stolen, AND you don't want to never encounter them directly. However, I think something could be workable and I definitely like the idea of enemies that you can relate to, and who have a similar power level.
I don't think bad guys failing to kill the main character is a plot flaw though. It depends a lot on how it is done, but we should expect that any villainous enterprise is going to have to cut costs and take risks, and the PC is simply the person capable of overcoming those risks quite well. Sarevok wasn't just hunting your PC, he was also trying to take over the city of Baldur's Gate. He tried to hire mercenaries, but you were too effective, and he's not going to jump the gun when he's got a lot of other villainous tasks. Irenicus really didn't have a lot of allies to spare in the first place, and he didn't consider you a threat at all, so the problem is that you're a loose end that failed to resolve itself. And so on and so forth. Maybe the major villain should not be portrayed as being so omnipotent? So, instead of you being the only thing in the way of this extremely well organized plot, instead the plot has it's own problems and the villains are portrayed as having to struggle to keep it going on track. He may still win if you don't intervene, but he doesn't give off a sense of omnipotence so much as determination and cunning.
In any case, in many ways I actually like the plot of DA2, where there was no overarching villain to fight, but rather a host of social problems and developments over time. Maybe a lot of people like the traditional grand narrative though of "big bad evil guy", but I really wouldn't mind this being stretched or deconstructed in a host of ways. Maybe even the idea of growth that you give, like the master villain is *not* as uber-powerful in the beginning(but still more powerful than your PCs) but is rising to power over the course of the narrative, and you are doing that as well. In any case, I'm probably digressing, but there are a lot of good ways to avoid the conventional villain, and I think those should be explored. (Maybe he is as powerful in relationship to you as Sarevok and Irenicus, but on some level he really likes your PC and would rather be friends, so he's handling you with kids gloves, etc)