Officially, there's nothing "really" stopping Nintendo from producing more Smash stuff (or rather, more Nintendo merch) through their partners. Jazwares has a pretty decent line of figures they've done previously in the "World of Nintendo" line and recently these special "non Mario" game series figures (thus far, BotW Link & Zelda, Samus, the Olimar and Pikmin set and Donkey Kong/Diddy Kong)
The amiibo "figures" could also be produced just by Nintendo, simply lacking a chip in them. I know a lot of people figured that Skylanders might do that one day, and Disney Infinity actually DID do that with their "Toy Box" series of action figures that resembled the Infinity style. Ideally if they cared, they'd just make new figures of the other future Smash characters.
Honestly I'm surprised they haven't made like updated sculpts of some characters like Link/Zelda that matched their Ultimate appearances. Or done something else like, make amiibo cards of ALL of the Smash characters, and alt versions as chase/rare cards. I could imagine them making a killing on that.