So, this whole Tom Nook discussion has made me a little interested in finding out just how likely Tom Nook as a character actually is. Seeing as how we now have an Isabelle Assist Trophy, it seems very unlikely at this point that Tom Nook could appear as an Assist Trophy himself. So that's one possible non-playable role almost ruled out. That leaves, I think, two possibilities. At this point, Tom Nook is most likely either playable, or a background character for the Wii U Animal Crossing stage. I decided to do a little research into what all we've seen out of the background of that stage, and I think I found some information that could prove interesting.
Looking through all the pictures that have shown off the Animal Crossing stage, it seems that there are four total groups of characters that can appear in the background: two groups for the town theme, and two for the city theme. Since very early on after the game was announced, we've known about two of these groups: one group is set in the town and has the characters Wendell, Sahara, and Pelly, the other being set in the city with the characters Crazy Redd, Dr. Shrunk, Gracie, and a Gyroid. In the vast majority of the pictures we've seen since, the characters in the background have either been out of the picture, or have been the same characters we've already seen the the first groups. Recently, however, two new groups of characters have been shown off. A new group of characters in the town could be seen in the picture where Rosalina was knocked unconcious; this group contains, as far as we know, the characters Sable and one of Tom Nook's nephews. Since this groups is smaller than the previous two, and considering that the picture of Rosalina offered only a limited view of the stage, it seems safe to assume that there are more characters in this group that have yet to be shown. The fourth group of characters was shown when the Cuccoo was revealed. This group is found in the city and contains (at least) Frillard and Labelle. Like the last group of characters, this group is likely incomplete due to the small amount of characters and the limited view the picture offers.
However, looking at the recent picture of Isabelle, it would seem that there is, in fact, a third group for the city. Each of the previous city groups had one character (Crazy Redd or Frillard) standing right next to the bust stop post. In the picture of Isabelle, there is no one there. What's worth noting is that this is the first time we've seen the stage at night, meaning that it's very likely that there will be time-exclusive groups. Seeing as how the city apparently has one, it's very likely the town also has it's own group of characters that only appear at night, and it's quite possible that both the city and the town have more than one group for that time.
The conclusion that I think can be reached from this information is that there is still room for Tom Nook to appear as a background character on the Animal Crossing stage. We've seen two groups of characters that seem to be incomplete at this point, and the strong possibility of further groups that only appear at specific times that we haven't seen yet. Based off of that, and considering that there are still plenty of important Animal Crossing characters we haven't seen yet, it seems very possible that Tom Nook could once again appear as a background character, and that we simply have not seen enough of the stage to know of this. Of course, it's also possible that Tom Nook will appear as a playable character. However, the possibility of being a background character is still a strong one as well.
If you'd like to verify this information for yourself, you can find this pictures of the Animal Crossing stage I referenced here:
http://smashbros-miiverse.com/stages/city-folk.
