First of all, I'd like to reinforce what others have said: using League of Legends characters seems like asking for trouble. Why discard the possibility of it being played outside of your circle of friends before you even know whether it's any fun? After all, if it doesn't work out then you may at least have gained in experience whether you use pre-existing characters or not, and if it does work out then why not allow it to run wild, unfettered by the threat of Cease and Desist letters?
Really, if you are any good with art, try to come up with your own chars.
And if you're not, then (as KingOfTheNoobs suggested, I believe) perhaps look for someone willing to help you.
All of that said, I think that it might work. My main advice is to try it: create a simple prototype (one character (i.e. a mirror-match), minimal artwork, a single arena, and a single minion type). Play it yourself, get others to play it, and see how it feels.
One thought based on the description: if you're not using platforms, why restrict your movement to a single scrolling line (if I understand you correctly)? Perhaps consider a 2D isometric arena in which the player and enemies can move around a little more, thus perhaps creating a greater range of potential situations. (Although perhaps that strays a little too close to League of Legends itself? I'm not sufficiently familiar with that game to be confident in comparing them, I fear.)
