I, too, would completely love it if someone were to create an entirely new shell for the DS with embedded calendaring functions.
Hell, borrow ideas from Acekard 2/RPG's GUI (like the top screen) and just add a few more options to make it a true portable tank. You can also look at PSP's XMB, iRShell and XMBController.
And then you can progressively add more features as you move along, hell it may even be able to fully replace the original DS OS like adding a better PictoChat...maybe even add a built-in PictoQuest game.
However, all I would really like would be a standard calendaring app, not too basic like DSO's, but not too bloated, either. If it behaved like Sunbird or Rainlendar, I'd totally love it. Unlike DSO, it would make full use of the touch-screen so I can add events by touching a date and write it out myself, DSNotes-style. Individual, but optional slots for time (e.g. 8:15 AM - Go to _______'s birthday) and an "overall" to-do list (as in, it does not need a specific date/time; useful for something you'd be meaning to check out or grocery lists and the like) would make it even better.
A basic, but smooth and appealing customizable GUI. Colors and DSNotes got it down.
But next to those would be a built-in option to soft-reset. I'm not asking for it to bring me to my cart's root menu (but if it's possible, all the more better), but just fool the DS into thinking I hit the power switch twice. It would make the portable experience much more seamless instead of going into the mentality of "once I boot into something, I'm stuck with that until I turn it off" - one of the reasons I don't boot into a lot of homebrew anymore.
...And that's it, I guess. I'll try to tidy up this post for increased coherency later. >_>