Yeah, the idea of Shia playing a student of Indy is a possibilty but for some reason I think the son angle is what they'll go for. The father/son dynamic is Spielberg's and Lucas' (to some degree, Luke/Vader/Anakin and his own relationship with his father) trademark, so I could really see it happening.
StwongBwidge, you said " if he is to be the son, then why does he need to be Ford-like in terms of bulk and adventure?" I agree Shia wouldn't have to be that way for this particular movie. But, if they by chance go ahead with more movies he'll have to embrace some of those traits for audiences to accept him as the new adventuring hero.