I still think the Sonovium is the end game, just not this specific instance. With how it's going, I expect one of these two tropey things to happen:
1) Lucian has forced the party into the Astral Sea to have them witness his great victory/ascension. He will command the obviously evil and lethal living city to not harm the party so they can witness his final ritual because he is super duper self conscious and he needs people to see his victory or else no one will know it was him and he did it so that for him isn't a victory at all. Lucian will continue to monologue some more and the M9 will strike him down right before his great victory because his Ego level is 21 out of 10 right now and he'll let his guard down at the pivatal moment to allow for an RKO reversal, cue dramatic death but the M9 finds out he achieved whatever he achieved and the Sonovium is unleashed on Wildemount, most likely because Cree actually did the smart thing and finished the ritual mid-monologue.
2) Little bit same as above except Lucian totally completes his ritual and awakens/unleashes/whatever he is doing with the Sonovium, but as he releases it he finds out he isn't a king like he's been monologing about, he's been a pawn all along, and is absorbed/killed/done away with by the very thing he thought he had mastered. The now unchained Cognoza Ward follows Lucian's memories to Wildemount.
The now Big Bad Evil City is loose in Exandria and is consuming the world slowly or doing some wacky shenanigans like bringing on the apocalypse , and the M9 have to negotiate with the major powers to get them to band together in common cause to save the day. Bonus points it pops up over the Menagarie Coast and roasts the area for the big "Chroma Conclave" turn and the M9 have to recover their famlies and see the devastation and stuff. Along the way, they discovere they REALLY needed the Cerberus wizards to 'Assemble', but Trent is still the leader and has taken the entire organization underground to try and take advantage of all of the worldly chaos. Maybe they stage a coup on the Royalty, maybe they just got MIA, either way the team has to make Trent a cadaverific corpse to get their help and boom Caleb is tied up.. Afterwards, after some choice diplomatic meetings with the Dynasty, they are on board to go Kord and declare war on the bad city, but a large force of crazy tentacle fueled cultists are invading one of their port cities and they cant spare the men. Turns out through...somehow, the cloven crystal was obtained by Avantika by intermediary as Uka'Toa and the Cognoza Ward had a fun telepathic tea time over their love of consuming and subsuming. Turns out this port city is Vandrin's home port and he is the biggest lead as to what the cultists are looking for, and Fjord and him and the M9 team up to stop that from happening, boom biggest hook#2 sunk.
Then finally, with the non-Tal'Dorei side of the world united, they march on the Sonovium and that's the endgame.