Okay here's an important thing to note. Mythics seem to be linked to deities and deities alone, as specified in the note for them. Thus, they must be linked to a deity, directly or otherwise.
Do note Dragon Gods, such as Naga, do count, since she is a goddess in a way. Now, unto possibilities.
Other than OC's, we have:
Tiki
Robin
Corrin
Those 3 are children to gods. One Dragon God, one Death Dragon God, and one Time/Illusion Dragon God. And they can be as strong as their parents assuming enough training. Tiki can replace Naga, Rob can use Grima's powers in one of Awakening's endings, and Corrin can probably grow as strong as his father assuming he doesn't go completely mad.
Now, the issue is, do they count the descendants of legendary deities? If so, then other units would be
Seliph
Micaiah
That's it
Now, I'm assuming the descendant of Legendary Heroes (who have the option to use some unique skill) are likely candidates (sorry Leif, you're relegated to Legendary status it seems)
Furthermore, another candidate is Roy, and I'll explain. Spoiler alerts here. The Binding Blade in a way caused the Ending Winter in 6, so it can be assumed that it's a god like weapon, and as such, perhaps he count? Not sure.
In short, who do you think will be a Mythic Hero (remember they have to be linked to a god and or children to said gods, can't just be any random bloke like Ryoma or Lyn)