I'm inclined to believe that Mario Kart will not be online, but rather, optimized for LAN play between Gamecubes. It just makes more sense. Nintendo does not have any online plans I believe. It was isn't a good business model. With LAN, Nintendo can get us all to shut up about online while we LAN it up with Mario Kart. I just hope they pack in the (possibly wireless) LAN connector with Mario Kart. That way, everybody wins.
Now, about these innovations... Shigeru was comparing them to Animal Crossing in an interview a while ago. I cannot begin to fathom what he's doing with Mario Kart. I'm hoping for more interactive, "krazy" tracks. Here's some of my ideas:
Donkey Kong Construction Track: Race through the construction site from the original DK Arcade! Zig-zag though the first level-ish design of the track (without latters, obviously) while barrels are rolling at you! It's hard explaining it on a message board, but it has the potential to be pretty cool if Nintendo got it right.
Pipe Land Track: Sort of like that Yoshi Cliff track on the 64, where there are tons of ways to get to the goal, but you have to find a good route. Some pipes will let you enter and exit, while those plant dudes will pop out of the bad pipes.
Super Mario Bros. 1 Track: I really liked the old school look of the Super Mario Bros. 1 stage in Smash Bros. Melee. I think it'd be cool if Nintendo could do something like that in the next Mario Kart.
Cloud Track: Now, all the Mario's usually have some sort of sky level in them, right? We need one in Mario Kart too! I would love to race on some clouds, a curvy rainbow, or even those busy (or buzzy?) beetles from SMB3! So much potential.
Castle Courtyard Track: One of those easy, first time levels. A circular track around the castle for the most part, but at one point, you'd bust into the castle and raise in a hall for a little bit.
I'll stop there, though I have a ton more track ideas. I think Nintendo should also include the 'old' 64 tracks in there for bonus stages (unlockable like in Smash Melee). Also, throw in a few non-looping tracks. Have a checkpoint at 3 different spots on a super long track. Bring back coins while you're at it Nintendo! Also, customization of the Karts would be cool. I originally posted this idea in the GA Forums. Collect a certain amount of coins to buy stuff for your Kart. Like, Nintendo-ish bumper stickers. Toad with a "The Princess is in another Castle!" sticker is hilarious. Wario with a "Have a rotten day" sticker would equally rock.
Lastly, NO SEGA OR NON-MARIO CHARACTERS! It's M-A-R-I-O Kart for a reason!