From what I remember, Vayne kills Lucian in Lore... at least that was the lore at the time- which would mean, she has to be within the roster.
Though, it would make a lot of sense to mix the corrupted star guardian storyline against Lucian because of the quote, "Lucian and his wife battled evil in Runeterra for years, bringing light to darkness and purging those taken by corruption." Which could essentially create a whole new line of skins to bring into league.
On a more serious note, I think it would be cool to play a "star guardian or odyssey styled" event for Lucian and his party. This could create a lot of opportunity for many more skins and profit for RIOT. For example, since these people are basically in a battle against the shadow isles, the enemies could also get skins. As another example, I'd use Yoric, Morgana, Shaco, Nocturne, Fiddlesticks, Karthus, Hecarim, Ellise, Mordekaiser, and Thresh as a base of enemy sources. In a constant race to reach the destination, you're constantly bombarded with shaco boxes, fiddlesticks crows (e), impactful moments of Nocturne's R "darkness", slowing barriers/wall of pain (w) from Karthus, Yoric constantly spawning his undead army, Elise spawning her spiders, Shaco Clones that are still dangerous, Miniature Hecarim horses, and Morgana creating a bunch of corruption spots that also act as source of temporary damage increase but in loss of hp, all in protection of thresh forcing such actions, holding him hostage. When a player dies or is about to, the game will use mordekaiser's ultimate (before the rework) and once dead in ghost form you must "kill it" in to revive your teammate from the party. Now onto the champions that would actually fill hour Lucian's roster.
Lucian's roster looks like it may be a mixture of Demacian allies/foes, and Shadow allies/foes. For example, Evelyn and Maokai (maybe even Elise) have a long history from being there and since they're both spirits with "objectives", I can't see them being on a team and more of a neutral party from which you barter with, but kalista... I can see Lucian becoming desperate or Kalista actually being a reincarnation of Senna. After all, Kalista has a legit story on her hands that can easily be read as Senna as one of her past life(s). With that being said, I can see each of these people constantly being reminded of their lore and feel like "starstruck lovers". On the other hand, I feel as if Galio MUST BE a part of the team because lucian is considered as one of Demacia's country men and Karthus is known to be a powerful mage which is identified as death. I can see Galio having a guardian (duh. lol) relationship with everyone of Demacian relations and a little more of a pathetic quirky "ha ha, you being alive is the reason why I'm alive" attitude against karthus. I was going to say fiora for shaco due to her place being a place for mannequins, but nevermind. I hate everything about Fiora and her lore. Although it'd make a LOT of sense, I hate her and that's just my opinion. For giggles, I'd say ivern. Just because the joke would be there for Maokai. But no. Not for serious gameplay reasons. He's be really bad. Braum would be the 3rd of the list for personal reasons, but also because he could actually offer lucian ascension from his past due to Braum's "strong heart" and personality. He'd step in saving lucian and offer the redemption of kalista who would be found attacking lucian during a love confession. For the 4th and final champion of the roster, Is you guessed it... vayne. Because no ONE ELSE FITS in better than she does... and I rather not care about detail on this part.