I was pondering today and I thought it would be cool if you could buy alternate lines, taunts, etc for your champions/summoner. Preferably lines that your opponent could hear too, that way you could show them off in-game.
For example, for Twisted Fate you could have a command like '/1' or just the button 1 shout something like "Test your luck!" at the opponent you currently have selected or the whole opposing team.
It would be an interesting way to make more flavor, banter with the other team, and raise the immersion level.
In general, buying more audio (music, effects, announcers, voices) with riot points sounds like another good way to spend points that wouldn't affect power level.