A lot of great suggestions in this thread. I wanted to add my 2 cents:
Improve the online lobby texting options:
Very simple fix. Go the Wii Sports Club route and let us preselect ones we make ourselves. It would also be great if we could use them against friends in the race during a particularly cool event (hitting or passing them).
Stat Tracking:
Competitive online loses some of its luster without this. I forget what all past MK's had but I'd like to know my best characters, courses, miles driven, etc
Disconnects/Connection Failures:
This has improved for me since I changed router settings but still needs addressing. Especially connecting with friends. If the lobby the friend is in is full, give that explanation instead of making me wonder what's wrong with my network this time.
Fix Battle Mode:
This would likely fall under free DLC instead of a patch but this is the one area of a fantastic experience where they phoned it in. Free pack of the SNES battle courses, please.
Map on TV Option:
Don't get me wrong, I love how they went for a clean look while putting all that stuff on the second screen. But I play off-tv a lot and when I do so, I'm at a minor disadvantage without a map.
Voice Chat In-Game (Registered Friends):
I know they don't want MK8 to turn into an annoying community of swearing youngsters, but why can't a registered friend and I chat mid-game? That doesn't mean you have to let non-friends hear what we say. Seriously, it just seems lazy when my buddy and I even play 1 vs 1 and can only talk in-between matches.
Never Stopping Item Rotation Wheel:
Only had it happen once but I got last place due to not getting to select quickly.
Next I'll have to look up fire-hopping (which will probably piss me off).
Oh my god the fucking rage of passing that very first line of item boxes and getting a coin. :/ All my skill just goes down the drain when some ass just spams the first item he gets and then I'm done. Once you get stuck in the "herd" as I call it (in other words, being unlucky enough to not get out front past the first initial row of item boxes), then your chances of winning greatly diminish. Essentially, the herd just launches whatever item they get immediately with reckless abandon and no strategy. So there's no real strategy there. Instead, it's just chaos. All the meanwhile, the person in first is totally free of the herd since they're just cannibalizing themselves.
Likely not your intent, but you gave a good argument as to why the blue shell is a necessary evil. The times I have gotten a bit of a lead in first, it turns into a time trial for me while everyone else is bouncing against each other for rest of the race. Haven't you ever been in a race where someone's far ahead and thought "man I hope someone grabs that blue shell, even a nincompoop can fire it successfully."