Personally I think it would be 
He is very Nathan Drake/Indiana Jones like and would really fit as the protagonist.
Who do you think it would be?
Also who do you think would be the main villain?
There wouldn't be one. It would be a mess for riot, "WHY ISN'T MY MAIN THE MAIN CHARACTER, FUCK YOU1!!"
Travelers would be good champions because they have the most flexible story paths
Ezreal, quinn, yasuo, lucian I could see individual stories following there encounters with other champions n such
I want 
because it can be cool to see what happen with him and 
.
EZ will be the protagonist 100%
Cho'gath is the protagonist for obvious reasons. The main villain is all the filthy humans and yordles that stand in his way.
obviously
Nobody
They'd switch from storyline to storyline often and not focus on one person.
the antagonist could be viktor maybe
