I don't know that people will ever agree about the DLCs. Well, people can't really agree about whether or not either DA game was good (hopefully we're reaching an agree-to-disagree state though).
As for the OP's suggestion - I don't know that Leliana as the main protagonist for DA3 would work, for many of the reasons already stated (those specific to the divisiveness). Personally, I would love to see another DLC with her - something longer than Song. I think she is a wonderful character with a lot of depth. Following the story of someone with her past (assassin, Chantry sister, companion, etc) is just full of all sorts of RPG-goodness. But again, my opinion.
I don't know that Bioware has made a direct statement about never creating a game with a single female protagonist. It could be done, and it could probably be done quite well (kick*ss female PCs have been coming in to their own of late and have been quite popular). It just may not coincide with their plans, especially not for the DA universe. Part of the beauty of DA, thus far, has been the ability for the player to mold their character. The PC is not static (unless in a DLC) and allows for the player to personalize the journey more.
I wonder though, if a DA game in which you changed main protagonists per chapter/act would work. There was a bit of that in the battle in Denerim section of DA:O, but having an entire DA game where you changed up...? I'd be interested to see it pulled off (and then we could have Leliana as one of the characters), but I think many would cry foul.