I was thinking E. Gadd too; he could soup up the Odyssey for time travel or make other (Gadd)gets for the heroes to use.
Of course, Nintendo likes to keep spin-off characters in spin-off games for the most part, but anything could happen.
The chances of this happening are incredibly slim, but what if there was time travel? Bowser could try to stop Mario during the events of Odyssey 1, while Mario navigated through past and future versions of returning kingdoms, as well as new ones. NDC could be a typical 1800s city in the past...