From what Corswain says in the short story By The Lion's Command
, from the anthology The Imperial Truth
. It would appear that The Lion is on his way to Ultramar.
It had been hard to watch him leave, but the seneschal had understood, as best he could, the reasons for the primarch's departure. Events unfolding on the Eastern Fringe could not be ignored, and maybe presented as much of a threat to the Emperor as Horus's own treachery. Or so the Lion had implied.
We already know from Savage Weapons
and The Lion
, that Jonson doesn't appear to support Guilliman and his plans for the Imperium Secundus. From this quote, it appears that he has taken it upon himself to go and see what Guilliman is up to, perhaps even with the intention of stopping him, describing it as a threat as large as Horus. So I'm thinking there's quite a good chance the Lion may end up appearing in the novel.
To make some presumptions and to extrapolate slightly. From Betrayer
, traveling out of the Ultramar system is apparently nigh on impossible for the loyalists due to Lorgars actions. We know Sanguinius and the Blood Angels are there, but we also know they will be on Terra, leading many to question how they would get there up till now. So my current theory is, the Lion will arrive in system, tense stand off, shouting will be had, much discussion, convince Guilliman to aid Terra, choose Sanguinius as the one to get there first and the fastest via the Lion giving his brother the Xenos warp jump...thing, he acquired to allow the Blood Angels to leave the system and get to Terra in time.
And just for anyone interested, though not what the short is centered on(nor is the above), Corswain has been ordered by the Lion to take the rest of the Legion and find Russ, whose whereabouts are evidently unknown. Also at the end it's revealed that he is splitting some of his forces under the command of Belal, their destination being.