He's already locked horns with Magnus in the Webway... but other than that... I definitely think a re-match with Lorgar is meant to be... but I think they are gonna hold off on that encounter.
The Word Bearers are relatively obscure... and I think GW will be pushing the sort of classic villains first and foremost. I'm thinking Mortarion is quite likely... possible Angron but I think Angron may be too powerful for Guillman to survive against.
So yeah, my money is on Mortarion. He's iconic, recognizable and most likely the next model to be released in the new starter set! lol