I say all of this as one of the biggest R&C fans ever...
As others have said, R&C has done spherical worlds before, since GC. Obviously not to the extent of SMG, but they have been there before. Really though, making R&C into a Mario clone will not make the series more popular.
If you look at sales of the PS3 games, you'll see that ToD has sold much more than ACiT. Part of this is no doubt due to bundling (as I believe ToD was bundled extensively), but also, at the time that ToD was released, it was one of the premiere titles for the PS3, because there were relatively few top tier titles at the time.
If you remember this ScrewAttack list of the top ten PS3 games in 2007... well, obviously they were having a bit of fun. But there's some truth to what they said. At the time, ToD was one of the standout games. By the time ACiT came out, there were tons of other games for people to choose from.
Really, the best thing for the franchise right now is to take a break. While I personally would love more games, everywhere I go, I see people complain about all of the games that have come out. And I can understand why yearly releases can get old. You see people get sick of Assassin's Creed as well for the same reason (for other reasons as well...).
Then, in the future, all they need to do is get more inventive with the weapons (seriously, a lot of the weapons in recent ones are basically just reskinned, renammed versions of older ones... not to say that there haven't been some great ones too, though) and keep the classic humour. I don't think that the series has gotten any worse at all since it began, and I think that ACiT is the best one so far. But it needs a break.
