Why not? Most of the other common projectiles have been used. You could throw in some ninja acrobatic dash abilities to make it fit the fantasy theme better. An agile knife throwing martial artist who can dip in and out of fights. I can imagine that such a theme would be very popular and sell well.
Shikigami3 votesKennen? Some people play him AD for fun.
XIII Vanitas3 votes
Also, we already have 2 knife throwing champions: 
Catastrop3 votes
Rıven2 votesDraven? Kennen?
Hjelp1 votesSeems like a good idea, but i think katarina fits the role pretty well xD.
intensablue1 votesTrue, although I was thinking of a sustained damage dealer rather than a burst assassin like Kat.