The only games I can remember that had "full version download play" are the Mario Party games and the 2 Bleach fighting games for DS. Therefore it should be feasible to pull it off again, regardless of roster size, as Bleach DS 2 had all 40+ characters available in download play.
However, I highly doubt that Ninty will do so for such a cash cow franchise as SSB, especially with 3DS sales having to mitigate the Wii U losses. What is more likely to happen is the psuedo-demo download play that they've used for the last two handheld Mario Karts.