Correct me if I'm wrong, but didn't Smash 4 have an unused Mario slot?
Either way, The idea of a second Fighter's Pass isn't that strange to me. Fighting games are kind of expected to receive long term support these days, especially if they are series that have large time gaps between major releases. Plus, Furukawa stated that extending the shelf life of major releases through DLC will be a major strategy for Nintendo.
The only reason I could see DLC stopping after one pass is because Sakurai wants out. While I'm sure he loves his job, he's been developing Smash consistently since Kid Icarus Uprising finished development in 2012. Given the fact that Sakurai is kind of an auteur developer, I could see him being frustrated by working on one series for so long. After all, that was the reason he left HAL and Kirby in the first place, he was tired of developing sequels.
Then again, he also stated that he enjoyed developing Smash for's DLC because it was a much more lax process than creating a full game, so maybe he really is in it for the long haul?