This is my thought. Could also be a character that is currently dead on WD. Someone heading to Atlanta.
I read that it was discussed on TD, but I watched TD last week and did not recall anything about a crossover. Plus, in the beginning they specifically said there would be no crossover between the two shows, so it may be just a rumor.
ETA: From what I read, there is going to be a crossover character. It was announced at NY Comic con. They will not say which character will cross over but here is what was said:
“There is one character that is going to go from one show, that I will not name, to another show, that I will not name,” Kirkman told the crowd of what has been a question fans have asked since prequel of sorts FearTWD debuted in the summer of 2015. “This is a huge event in the world of The Walking Dead,” he added of the crossover expected to occur in the next year.