Hm. Indeed, Dawn's kind of a tricky one. She doesn't have any pokemon whose stories could easily be wrapped up in the space of a two- or three-parter or whatever like Misty did with Togepi (well, besides something like Piplup deciding to evolve after all, or something), and Isshu doesn't have any contests (though musicals may be just enough of an analogue that it might not matter).
The way I see it, here would be the most likely possibilities.
One: like I said, musicals may be close enough to contests that it may not matter. Of course, this also assumes that we have a main character even doing musicals at all. Personally, though, I wouldn't be entirely too keen on a second former girl coming back just to lose to one of the new main characters.
Second: there ends up being an event in Isshu that's similar to the Wallace Cup--basically, a PR move by whoever in an attempt to popularize contests in Isshu--and Dawn shows up to participate. All of our heroes get involved in the competition, which (because no one else in the group would be working towards the same goal in the long run) would end in Dawn's favor.
Third: It was said that the Isshu saga is supposed to be faster-paced, right? I'm tempted to entertain the possibility (and it really is just that at the moment, make no mistake) that our heroes will get an interlude saga a la the Orange Islands and Kanto Battle Frontier. Assuming such a thing doesn't take place in Isshu (but rather in one of the original four regions), it wouldn't be much of a stretch at all to include Dawn in a contest arc similar to what I just described.
Of course, those last two also assume that we'll be getting third-generation remakes along the way. Really, if we do, do you really think the writers would actually pass up a chance to advertise one of the things that helped to re-popularize the show in the first place?