Probably unsurprising, but it seems clear that this game will be getting the Mario Kart 8 treatment.
Proof of DLC.
The following text appears in the Sm4sh Rom Dump:
Set the stages that won't appear, even when selection is set to random.
Turn all stages off.
Make all stages available when using random stage selection.
See stages you have downloaded.
See all regular stages.
Select OFF to exclude this stage from random stage selection.
Select ON to include this stage in random stage selection.
It matches the stage selection in additional rules perfectly.
Also note the notice the missing square, just like in the stage selection screen. That's where you can select the DLC.
I originally posted this on Smashboards and then on Reddit.