I could see Nintendo wanting to treat Smash more like an ongoing service for a few years, but on the flipside, I could also see them wanting to give Sakurai a break, and Sakurai being unwilling to relinquish control of the Smash series over to someone else. The latter would mean that DLC totals at 5 newcomers (and a free potted plant, haha).
But if the former happens, and Waves 2 and beyond become a thing, then there's no way we come out on the other end of it without at least a few playable Assist Trophy characters. Surely not...
But that won't be for a while. Currently, I'm just bracing for the release of this game and the first wave of DLC, which I have a hunch will be an onslaught of more third-parties. I'd love to be wrong, but I was woefully optimistic about other aspects of this game, and got burned in return.