They'll probably try to figure out some way to help Grey have some physical form since he's clearly more than just memories and personality in the Trill symbiont and Culber being a doctor will try to figure that out.
The Voyager Doctor's mobile holo emitter is already centuries old by this point. Also I didn't see any indication Su'Kal's ship was necessarily irreplaceable holographic tech wise, so I'm not sure why Starfleet can't pull up the specs of that ship and use those same holo emitters to give Gray a new body.
As for Season 4, I can't think of any bigger way to smash up that optimistic Season 3 ending than having the season premiere open with a full Borg invasion of Kaminar, with all Ba'ul and Kelpiens including Su'Kal assimilated and Saru barely escaping.
The Borg, having incorporated Su'kal's biological distinctiveness to their own, cause a new Burn that cripples the galaxy again, and then go on an assimilation spree.
Panicked, Admiral Vance orders the forcible conscription of all Kwejian to power the Federation's new spore-drive fleet to stop the Borg. Book, angered at the Federation's sudden betrayal and enslavement of his people, goes rogue, swears revenge against Burnham and the Federation, and steals the SB-19 data to give it to the Klingons.
Craft shows up out of nowhere and tries to destroy Federation HQ (he's at war with the V'Draysh remember), but is only stopped by the appearance of Discovery, which he recognizes. However Discovery and the sphere data have no idea who Craft is (wibbly wobbly timey wimey), leading the crew and Vance to realize, to their horror, that to preserve their timeline they have to break the ban on time travel, as they now know Discovery has to be sent back in time 1,000 years to explain Craft's recognition.
Vance reluctantly allows Discovery to be sent back in time. However, an assimilated Detmer (who looks like a Borg anyway) was aboard when it was sent back, and Detmer thus becomes the first Borg queen (meaning the Borg have no origin and in fact are a temporal ontological paradox).
