Thanks everyone so far whose replied.
The RPG's...I didn't mention in my original post. I really don't know if they would be considered proper sequels or spin-offs. They definately serve to enhance the Mario universe in ways the Mario Party or the Mario Kart series couldn't and they are all expansive games, some with more platform elements than others.
I will say the one element that is very different in the RPG's is the character of Bowser. He's way more similar to his cartoon/comic persona, and as a recent example, when I played Super Paper Mario, Bowser was portrayed as this inner-pure heart, deep-inside hero, *giggle*, ect. then in Super Mario Galaxy his first scene depicts him terrorizing Toad Town in ways far more graphic than any in a Mario entry. It makes Bowser seem...schizophrenic or bi-polar. He saves the multiverse...then wants to conquer the entire universe?
And I didn't realize Super Princess Peach wasn't a Nintendo exclusive. It certainly seems like a sequel game, just a role-reversal. Hmmm.
Personally I don't see why the Mario Land/Wario Land series wouldn't be. When I was very young, nobody took the GameBoy
too seriously, but I think the games have stood the test of time and it's even crossed over to the GameCube (If you consider Wario World part of it) and now the Wii.
Oh, and as for Super Mario Bros. 2...I think that game is now firmly established in the mario mythos. Can you imagine the Mario universe without Bob-Ombs, Pokeys, Shy Guys, Pidgits or last but not least Birdo? The series hasn't ignored SMB2.