You can only replace a regular ability for a regular ability, or an ult for an ult. What would you do?
with 
Valkyrie on his E.
I am a giant full ap void lizard flying at you dropping bombs to cc and oneshot your face. Deal with it.
I'd replace Alistars ult with Yasuo's ult...
E with 
q
Singed passive with Veigars passive
ult with 
idk i think it would be fun
E with 
E
seems like it would fit the bear better idk
slow pool replaced with Galio's shield.
He now does damage by getting chased and gets healed by being hit.
any ult with karthus ult
Urgot q with ez q but still have the e lock it on
Tryndamere q with ezreal e. Now I have 2 gap closers and a slow
's W with 
's hookshot. You thought you could escape his poison? You thought you could catch him? THINK AGAIN!
's R with 
's R. You cannot kill the million-stack doggo.
's W with 
's E. She now has ALL of League's mobility, and you can't do anything about it.
's Preparation (or 
's W) with 
's E. You now cannot stop her (them).
's passive with 
's passive. He now is the million and twelve stack doggo. (For moar rage, give him 
's passive)
's R with 's R. They can actually ult in a team fight without saving the enemy team, now! And they can actually survive a fight!
Not an ability, but give 
a Scuttle crab skin. His play rate will spike to around 70%.
Just imagine
E becomes E.
I can see the combo now... E to stun, W,Q,P,R, ded.
With 
E for his e
Hyper mobile juggernaut with 350 ad incoming.
passive for 
passive (little flowers instead though)
or if passives aren't allowed
q for 
q
's E with 
's W.
