If you're curious who the biggest contenders for Primarch returns are, here they are:
Fulgrim is almost a certainty. Slaanesh is preparing for a return in 40k and in AoS, and many credible sources have said that Fulgrim's model is already sculpted.
(There's an interesting, but unlikely theory that Fulgrim will return as a daemon and a loyalist. There's a clone of Fulgrim out there who has all of his memories but has condemned Fulgrim's fall from grace and is still loyal to the Emperor's values)
Leman Russ is a possibility. There's a bunch of Space Wolves who were lost in the warp during the Heresy returning, including those who fought alongside Leman Russ. Logan Grimnar is searching for Russ, although his return would be bad as it would signify the Wolf Time.
Jaghatai Khan is also a possibility, although I don't know how popular of a choice this would be. A bunch of White Scars went into the webway to look for him and the recent Dark Elf codex has a sprawling artwork of White Scars fighting Dark Elves in the webway.