Villager talking about Dr. Shrunk.

[Serabee]

I've seen that book mentioned before for sure. I don't think it's necessarily a sign that he's coming back as much as an Easter Egg.

...That being said, Nintendo has confirmed to have a couple years worth of updates prepared. I'm willing to bet he and, basically, all the other currently missing characters will be back at some point. When? I have no idea- but eventually!