One popular theory has the character of Abraham Ford (Michael Cudlitz), who spent several seasons on The Walking Dead before dying at the hands of Negan in the season 7 premiere, showing up on Fear the Walking Dead (which takes place at an earlier point in time) when the show switches its location to Houston.
Yet another theory has the character of Morales, who disappeared during season 5 of The Walking Dead, re-emerging on Fear.
Given the way the two shows’ different timelines work out, the easiest and most logical thing is still for someone from The Walking Dead to show up in a brief appearance on Fear the Walking Dead, and the most logical character to make that leap is still Abraham (regardless of what Greg Nicotero says).
The Walking Dead executive producer Greg Nicotero has been listening to fan theories about the coming crossover between the venerable AMC zombie series and its sister show Fear the Walking Dead, but though he admits some of the guesses have been intriguing, he says fans so far have missed the mark.