As said in the title, themes would be a great addition in my opinion. Animated themes? Even better.
Also, It is kind of disappointing that there's no music or no start-up logo animation. (As iconic as the Gamecube let's say.)
It'd also be nice if you were able to categorise the games you own on the dashboard. Just to make it look a little more..."neat."
There's a lot Nintendo could still add but I doubt it'll happen. Especially this far in. I like that's it's straight to the point but it'd just add a little more something if they got creative with future updates.