Worlds will collide!
The most plausible crossover would be if Rosita shows up in Fear The Walking Dead. We know she says she hooked up in Houston with Abraham after being with a few guys/groups prior to meeting him, and we know she said she moved around with them a lot before that. So what if Rosita started out in the Cali/Mexico region, crossed paths with the Fear cast, then hooked up with a guy heading east. That would ultimately lead her to Houston and Abraham, which would finally lead her on her mission to DC to deliver Eugene.
It’s unclear any of the The Walking Dead cast could get all the way to the west coast so fast, and we already know Rosita was going in an eastward direction, so… I think Rosita being in Fear makes the most sense considering the timelines of where each show is. Anything else would seem to forced in my opinion.