Dark Samus and Chrom still made it thanks to poppularity despite having non playable roles in smash wii U, we can make it again.
As for characters going from non-playable to playable thanks to DLC in the same game being impossible, just because something hasn't happen before doesn't mean it cannot, smash has made a habit at taking assumed pattern and destroy them :
-Look at 3rd party characters,
-Capcom getting a 2nd rep with Ryu,
-Ridley,
-Little Mac making it in Wii U despite being an assist in Brawl,
-Charizard after being a pokeball pokemon in 64 & Melee.
If you give up then yeah, it sure won't happen, if you don't, then it just might. There's nothing to lose except time, time that we have in spades until Ultimate's release.