Take any two champions and swap lines
One boring day, I started wondering, "You know what would be interesting? What if Mordekaiser and Veigar swapped voices?"
Mordekaiser's introduction in Veigar's falsetto sounds absolutely hilarious and in character, while Veigar's threat in Mordekaiser's rumbling, deep voice sounds chilling and badass, and the rest of their lines are rather fitting as well.
Anybody got other ideas for awesome and funny voice swaps?
<---> 
<--->
<->
"Misery has a delectable taste."<->"That tasted purple"
and 
. Diana's lines would be more prone to acceptance, defending her beliefs, and whimsical jokes, while Leona's would be more so brash, warrior-like, and would show how much she would love to bash your face with her shield.
and 
. Fiora would be less prone to bringing death, and would duel with grace and without killing, rather she would spare her opponents, unless the need to kill arises, while Riven instead of exiling herself, would be staging coup's and trying to take peeps' heads off.