I too, would lie to see an older Doctor in the future, but, personal anecdotal experiene has shown me that there are quite a few fans of NuWho who can't get into the Classic Who, "Because the Doctor is so old". So, I'm not sure if the General audience for NuWho would accept an older Doctor, we may be stuck with 20-something or early 30-something Doctors for some time to come
(Again, this is anecdotal, not meant to be preseted as fact, so, it may be that my personal experiene doesn't translate to the General Audience)
Well Eccleston was over 40, and Tennant was, I think, 37 when he left the role so I think people would be fine with another Doctor around that kind of age--and really mid 30s to early 40s would seem to be an optimal age for the role anyway if you look over time.
Paul Bettany. While he's a "name" in the industry, I would argue that he's not a "big" name... He's not done well as the lead in his own vehicles, box-office wise, but I think he could carry the part of the Doctor...
I think Bettany could be a great Doctor, but having listened to him interviewed on the Kermode/Mayo show the other week it's clear that, a bit like Eccleston, he is an ac-tor, and he has no interest in doing anything big budget (not that Who is big budget) and has basically walked away from blockbusters to concentrate on indie films that appeal to him. Also he lives mostly in New York so whilst he might come over to the UK for short stints of fuilming, I'm not sure he'd want to leave/uproot his family for 9 months of the year, especially given his other half's an actress (Jennifer Connolly)