As someone said above, and I'm sure many have also said this, the Dragon Age series is about the world the writers have created, not necessarily about a certain character, like the Mass Effect series. So it makes sense that each game might (not necessarily will) have a different main character. Trailing from this, every game so far has the character support the story, not the story support the character. If the developers were to use either Hawke or the Warden as the main character for the next game, it wouldn't make sense and wouldn't fit with the trend of the series; it's all about developing the conflicts in the realm of Thedas. The first game focused on the darkspawn problem and the fifth Blight, while DA2 focused on Kirkwall's role in sparking the Chantry civil war that now plagues much of Thedas. All this being said, however, it does not mean that both characters will not play a role in the next game(s). I believe that the epilogue of DA2, which mentions the Chantry's search for both Hawke and the Warden, isn't just put there for nothing. I think that, while there will be a new main character, Hawke and the Warden will be involved with the DA3 story, in some way, shape or form. I also think that Chantry Seekers will be in the center of the DA3 plot, as they are the true Chantry power due to the mutual fear/respect of the Seekers between the Templar Order and the Circle of Magi.
Now, this being said, how would Bioware go about incorporating both Hawke and the Warden into DA3, technically speaking? Hawke wouldn't be too hard to incorporate, as his/her personality and looks, not to mention choices, are recorded by DA2. However, the Warden is more difficult to handle, as DA:O is older and didn't have what DA2 had. But here's how I see them fixing that:
Option 1: Bioware releases a patch for DA:O that would record the personality of the Warden, and as such, would be able to be imported into DA3. This causes many problems, as those who have already completed the game wouldn't be able to take advantage of this. Also, Bioware releasing anything for DA:O seems highly improbable at this point, since Witch Hunt was stated as being the last bit of DLC for DA:O. However...
Option 2: A lot more interesting. Either as an expansion of DA2 or as part of DA3, Hawke/New Character meets up with one of the Warden's companions (most likely Morrigan), and in a style similar to Varric telling the story of Hawke, Morrigan will retell the story of the Warden. Of course, it won't be an entire game like DA2, but would go through the major plot points of DA:O, and be enough to establish the Warden's race, gender, backstory, and personality, which would then be able to be imported into DA3. It sounds a bit farfetched, but it could work. An issue arises as to how the Warden would be voiced. People seem against the idea of having the Warden fully voiced, as it takes away from the personalization of the character. However, Hawke is fully voiced and people don't seem to have a big problem with Hawke. You would have different voices for each gender and race, but the Warden would still be fully voiced. I don't know, I just don't see the problem with having a fully-voiced Warden.