It's a bit uncharted for DLC here. Historically (yes, I know it can change), DLC falls mostly into one of three categories: Veterans (Not applicable for Ultimate unless maybe they give us something like a OoT Link echo which I would actually be in favor of), Third parties (We've seen plenty of em), and essentially ads. Now, you may be wondering about Plant, but the evidence is there to suggest Plant was mostly a late addition. There seems to be some thought process going on at Nintendo that it either needs to be third party or currently relevant to sell. Now, the pass might change that, but it didn't seem to this time. I imagine the first fighter of pass 2 will be some third party (probably not happening, but Geno might do the trick) or maybe more to get people to buy it. At least, maybe if they planned for this kind of reaction. However, after they get plenty of people to buy it, it could theoretically be "safer" to pick some other choices. Such as Rex.
Edit: Spirit Tracks? It has a stage, but Toon Link is more Wind Waker.