I'd say there's certainly a lot left for a sequel!
http://www.mariowiki.com/List_of_items
http://www.mariowiki.com/List_of_enemies
Of course they won't go overboard with that stuff to overwhelm the players. But I can also still see more themes because they need them. Also a water block would be nice too so that we can make ponds of water in some places, and also give us a theme that allows water only on the bottom of the screen so we can have some Boss Bass trying to eat us as we're hopping from land mass to land mass.
There's still a lot of things to explore and new gimmicks to add as well, maybe specialty themes relating to other Nintendo series.
Oh, well, yes, I certainly can't deny there are lots of individual items that could be included. There are always more themes, more items, and more enemies. However, I suppose what I'm saying is that I can't think of any substantial ways of iterating on the game. Everything that comes to mind is something that gets tucked into an existing menu. Like, I could see an NX port adding a new row of tools to build with, one or two new environments, and maybe a new game skin (Yoshi's island or Mario 2, perhaps). But all of that seems perfect for an enhanced port rather than something I'd call a sequel (kind of like a Hyrule Warriors Legends level of expansion). If something like the ability to string levels together for actual worlds or a full-fledged music creation mode were included or double the additions I mentioned above, then maybe I could see why a sequel rather than an expanded port makes sense.
In contrast, Splatoon to me could inevitably have those incremental improvements (more weapons, gear, and stages), but it could also include big changes: new multiplayer modes, much expanded local game (co-op challenges? four-player competitive?), a new single player campaign with new mechanics, etc. I guess that's why I see them differently.