In the book Elemental Council its repeated a couple times that the Tau are planning on attacking Ultramar. Farsight vs Guilliman is one of my favorite matchups since its the author of the codex vs the mirror codex. I'd love to see it. To see his codex in the hands of an alien, to see that one of the greatest students of the codex is an alien that's reverse engineering it. I think it'd be neat.