What if Senna were to find a way out? Her drive to erase all the shadow isle inhabitants even stronger then before? Maybe to the point where Lucian can't even recognize her (Slight change in appearance of course, Personality change great though.) What they could do with her would be pretty cool. Though this may be impossible it's just a idea. I'll post a picture. 
I don't take credit for none of the picture I post.
I feel like this would invalidate Lucian's drive and his whole character. Like, where it is a cool idea, no doubt about that! All it does is fill up a hole in a characters heart. Heck, a CHARACTER DEFINING hole. But, I do dig the idea.
It's just in a game like League of Legends, you have to make condensed versions of personality. But, you also have to have them be interesting and have a reason or purpose that explains why they are here, for Lucian. It's Senna's death (as well as his whole "CLEANSE THE SHADOOOOOWS" sort of thing) that drives him to do what he does. So, if Senna was to show up, He has no reason to fight anymore. It's just a bit too much character interaction than League is set up to handle.
Instead of making this Sienna, could instead go with Lucian and Sienna's daughter (but slightly younger). While Sienna did die, it could be written in the retcon that she was pregnant before she died or had given birth prior, the child could have been raised by someone else as Lucian was off getting revenge.
