i think the buddy platformer, to coin a phrase, is something rare basically invented in the late 90s with banjo kazooie and which was ripped off over and over (jak and daxter, ratchet and clank, etc.). those clones did well enough, and ratchet and clank is making a comeback (though given the reception of lbp3 i'm not sure it will be greeted with exactly the same kind of fawning praise sth like up your arsenal got back in the day).
the market however has moved on. 2d platformers routinely outsell 3d platformers (blame nintendo?). 2d platformers have less of a tendency to become tedious collect-a-thons, as well, which was one of the problems both with banjo tooie and with donkey kong 64. basically i don't see a 3d platformer doing the kind of figures banjo did. the decline from banjo kazooie to banjo tooie was pretty stark..
