I like the general direction of your thought, but I’m not sure about the details. Zovaal wanting Tirion as the new Lich King? Sure, I can buy that, it would rather neatly retroactively explain why Arthas was so obsessed with the idea of capturing and turning Tirion during all of Wrath, even before he took up the Ashbringer. Remember, the stated reason Arthas gave for his willingness to sacrifice every single death knight in Acherus to the fury of the Light and the Argent Dawn was to draw Tirion out into the open…which is a bit odd for Arthas, since he didn’t have any real personal history with Tirion prior to that point. If that desire to turn Tirion was the result of Zovaal’s influence bleeding through, that would explain a lot.
What I’m not so sure about is the idea of Tirion leading the Four Horsemen and the Dreadlords using the Ashbringer. The dialogue and quest text at the end of the death knight order hall campaign strongly implies that Tirion was never actually meant to be Bolvar’s preferred candidate to lead the Four Horsemen, but rather that the entire gambit of going back to Light’s Hope to raise him was meant as a test for Darion, Bolvar’s actual preferred choice. Like Arthas before him, Bolvar knew that an attack on Light’s Hope would fail, and moreover knew that Tirion was probably the last person on Azeroth Darion would want to see raised as a death knight aside from his own father (and thankfully the location of Alexandros’ mortal remains is unknown), but he wanted Darion to prove his commitment to the cause, even in violation of his own personal ethics and in the face of certain defeat, while also showcasing both that commitment and his willingness to self-sacrifice to the other Horsemen, thus earning their respect. The third attack on Light’s Hope was basically one giant horrific team-building trust exercise inflicted on the Deathlord and the Horsemen by Bolvar, because Lich King Bolvar is a jerk who can’t be bothered to communicate directly and just ask Darion if he wouldn’t mind dying again so that he can once again follow in his father’s footsteps.
As for the Ashbringer sword, I don’t know that the Dreadlords, or at least the Warcraft III ones, could handle an uncorrupted Ashbringer directly (Lothraxion probably can). I don’t think it was the Ashbringer that allowed Tirion to escape Arthas’ trap at the top of Icecrown, but rather the Light itself, and if it’s the Light that Zovaal needs, then that would explain both his interest in Anduin’s Light-wielding abilities and why he’s kept Anduin alive. That said, the Ashbringer is still notable for being the blade that shattered Frostmourne, and that alone might be enough to pique Zovaal’s interest. I’ve mentioned before in a different post that I initially though that Alexandros being captured and imprisoned in the Maw was just Vyraz making sure that he wasn’t around to get in the way of his schemes in Maldraxxus, not unlike how he was targeted when he was alive for being in the way of both the Dreadlords and the Scourge, but between Alexandros being captured and tortured and Denathrius’ experiments on a Naaru, it’s possible that Zovaal might have enough information to make a weapon similar to the Ashbringer. For that matter, it’s also worth noting that Shadowmourne, which in theory was part of the winning battle against Arthas and helped counter Frostmourne in the same fight that saw the Ashbringer shatter it, was conceived of and forged by Darion, and that Darion was also captured and tortured in the Maw. I don’t really expect Blizzard to actually go that direction, but if Zovaal thought that Frostmourne was the Best Sword Ever, then it would make sense for him to be curious about any weapon that could counter its power or, indeed, shatter it. Perhaps such information would have been useful in making Kingsmourne, even.
I would love to see Tirion again, but if we see him come back for something like this I would demand that Alexandros also be involved so that they can chat about having to break a young man out of mind control from the forces of Death again while Darion grumbles in the background trying to ignore them. I don’t know what it is that Blizzard has about turning virtuous Light-wielding young men in their late teens into mind-controlled instruments of Death who wield their fathers’ corrupted legendary swords, but it is a little strange that they’ve done it twice now.