"Lucian has Senna's weapon.." True; however, in the stories you read that these weapons were hand-crafted via certain materials and specific rituals; if anything, Senna would know how to make new ones or get hers back and Lucian still be double-handed with a new one. I mean, GP lost his ship but somehow still has his ult(???) People said it's the Noxian battleship he stole from Swain that he's using but there was never a confirmation on that (besides, gameplay and lore don't always have to go side-by-side 1:1, even Ritoers themselves have said so)
"Senna is dead, her soul is in Thresh's lantern..." Now, aside from the obvious self-contradiction and the oxymoron in that very sentence, there are many instances in the various stories where you surmise that there's still hope; how was she able to help Lucian? She actually spoke to him, it wasn't him just imagining it. And who tells you there isn't a magic strong enough to restore someone back to life (the way he/she was) in the whole of Runeterra? Maybe that's why Lucian left Demacia and wanders around the world (other than to extract revenge)?
"would ruin Lucian's lore..." Not at all (imo at least). The Ritoers behind the lore (world-building whatever) team themselves said "many times, the setting and the characters feel frozen in time, never move forward. We heard you and are looking to change that." Now, how many of Lucian's stories end in the notion "it's Thresh season!"? Lemme answer; all of them (even the alternate universe one with wild west skins). Same boring thing every time. Now if Senna was to come back, Lucian would find peace, redemption, vindication. Also, this would all but make them stop their efforts of hunting down these spectres and all other undead abominations, making them work with extra zeal so that none would suffer the same fate. In addition, Thresh would now have an actual part and say in this vendetta (the one soul that was "stolen" from him) cause, as things stand, Lucian is just "another satisfied customer" for him, he only views him as another subject to draw amusement from his despair and/or a potential new victim.
Now, aside from the gif in the champ roadmap (with very strong shadow isles and light powers/purifier vibes) there's also the special interaction of PROJECT Pyke with Lucian; while some may be quick to discard it as "wow, another BM to Lucian about Senna, what's new" it may actually be teasing her, new champs have been teased like that before (like Swain teasing Bae'Sa).
Personally, I'm of the opinion that Rito should slow down on new champs, there are already more than enough and instead focus on VGUs/smaller scope reworks and updates cause the longer the wait, the longer these champs are being left to rot, but if it actually turns out to be Senna, color me super excited and intrigued (and I bet they know she's gonna be a big seller too; and nowadays it's all about that sadly).