Sure there'll be new IPs, but at some point we're going to run out of super hyped characters regardless. Xenoblade is the most popular new IP Nintendo has created since Pikmin possibly, and Shulk is still only somewhere in the middle in terms of requests. The characters that are currently middle ground hyped will suddenly be considered top tier next game, and so forth. Unless Nintendo can spit out two or three franchises that are as popular as Pikmin between now and the next Smash Bros, guys like Isaac and Bandana Dee (who I both support mind you) and whatever new characters might be introduced in existing franchises are going to be top tier for next game. It's already been visible this time around if you compare now to pre-Brawl. Ridley was ~the 5th most wanted character, behind Sonic, Dedede, Diddy Kong and Olimar, but since he missed out last time he's by far and wide been one of the most requested newcomers for this game, his only competition being K. Rool and Mega Man, and Ridley's support has been fairly consistent since so it's actually a decent comparison. Mega Man and Little Mac are already in, getting rid of two of the top tiers already, if K. Rool, Ridley and Palutena are in too we're going to sit with Dixie Kong or someone like that being the next game's Sonic/Mega Man, which sounds pretty bizarre.
Eventually we'll have run out of classic Nintendo characters to pick from, and I don't really believe that Nintendo can crank out a bunch of Pikmin-level new IPs constantly until the end of time, and they really have to be that popular (and even then Pikmin doesn't really offer multiple choices for characters), because a Wonder Red type of newcomer isn't going to sell many people on a new Smash, no matter if Wonderful 101 is good or not. Of course we're going to see the inclusion of more 3rd parties before going all the way to guys like Goku, but I won't be surprised if in 2028 or w/e if we see a character in Smash, who didn't spawn from a video game initially.