I think it's useful to define "ambition," we could be talking about length of development, budget, design, graphics, the whole package, or any other criteria.
I don't really think you can say Galaxy is not ambitious. You can say it "perfects" 3D platforming, whatever that means, but achieving perfection (or attempting to achieve it) is about the most ambitious thing anyone can do.
As for the future of 3D Mario, it's likely going to be an evolution of 3D Land. They're getting the perfect level design and innovative gameplay mechanics/level gimmicks down to a formula at this point. 3D Land's problems, I feel, are the result of a much smaller budget and development timetable than either of the Galaxy games. But I think it's reasonable to say that the next console Mario will be back in the Mushroom Kingdom, just with space levels that will remind people of Galaxy superficially (whereas all the levels and mechanics will be evolutions of those found in Galaxy anyways.)
But who knows, they could surprise us.