Since you are adding new champion mentioned in other lore like Senna .
General du couteau the father of katarina (and not confirmed 100% father of cassiopia also ) and the one who defeated and adopted talon deserve to come back .
From the lore of Talon he can be a assassin and from Katarina it said he went missing after going for a mission .
Plus in all the champion roles there is no relation between 2 lol champion that is parents and kids , there is brother (darius and draven) there is sisters (morgana and kyle)there is husband and wife (trynda and ashe) but no parent child relationship.
I dont have a idea for his spells but i have ideas for his role and chat interatcions with other league champion that for me i think they are really cool.
First the reason for him he went missing it can be cause he went to inspect this void gate and find himself vs a new void creature that can trap himself and others in a void dimention in like a iternal fight (this void beast can be used as a futur champion ,he can be used as a teaser when a champion go disabeled for a bug for some days you can type "disabeled captured by The void beast " if a void champion is disabeled just say " disabeled went back to the void to relax" :p)
After all this time traped he escaped the void beast and went back to his home noxus .
Fun intereaction with other champions in lol:
General du couteau :it has been long time since i saw this pretty face
Katarina : oh thanks dad .
Katarina : mmm .. dad ! I meet this boy ...
General du couteau : Oh he must be a fine noxus warrior i see .
Katarina: mmmm... yeah he is a fine warrior.
(Nice reference to hidden relationship between kata and garen )
General du couteau : to my eyes i only see this pretty littil girl .
Cassiopia: you really think i am still pretty dad ?
General du couteau : i took you under my wing cause to me you were like a real son .
Talon : thanks master.
(Or he can reply thanks father but he still look up to him cause he is the only one who defeated him )
General du couteau(to katarina): i miss this old days me and you cutting enemys in half.
General du couteau :do you have enaught daggers before we head to fight .
Katarina: yeah dad ! dont worry.
General du couteau (to swain) : i see you have been running this place really good in my appsence .
General du couteau (to darius) : ohhh my old friend lets see who get most kills.
If you like my idea for General du couteau to come back in the lol universe please vote me up so the developers see my idea.
And i am sure you also have cool interactions ideas write them down in the comments.