Hmm, well, they did use Celebi as a device to allow the time travel and that to occur in the first place, and I doubt they'd use that again.
It' true, Dialga also has relations to time, but it does more so uphold it than travel through it? Also, that, he was already in a film, and I don't think we'll probably see much more of Cynthia again, really.
It's an interesting idea, though, despite having its connections to what happened previously, but I'd say it's very unlikely. The idea of a champion having affections for Ash in their young age, is interesting. If it happened, would you like her to remember him as an adult or not?
well consider this dialga sends ash back in time during the arceus movie so a romance of ash and cynthia in that format is definatly plausible.