Well, they better show us something new.
What with a whole tournament, it would be hard for them to avoid.
On that note, I've been thinking about the tease and the tournament for a little while, and the lingering doubts as to whether this is a definitive edition or an all-new game, and Sakurai's love of keeping secrets (dojo notwithstanding). If Smash 2018 is just a port, then Nintendo can just show everything, no worries. What to do if it's a new game though?
What would be interesting is if Nintendo plans to take inspiration from Splatoon's content rollout plan: launch with a smaller, balanced core cast of fighters, all unlocked from the start, and then release characters and stages periodically as free DLC. For Nintendo that would mean a smoothed-out development cycle with money coming in even as the team continues to work on the game and a guaranteed holiday 2018 release. For players it would eliminate the unlock grind, while still preserving the excitement of "A new Challenger Approaches." The only downside would be for non-connected players or those with really limited storage, though I guess they could also go the Splatoon 2 route and release a "Starter Edition" a year later with all the content so far on-cart.
Seems like a win-win to me, and we all know how hot gaas is right now. Then again, with what Nintendo charged for post-release characters on Smash 4, free DLC might not seem as appealing at Nintendo HQ. Here's hoping they don't try for paid seasons.
EDIT: Now that I think about it, the Smash Dojo could also take on a whole new role instead of dropping dead on release day.