Just a thought... I totally get the desire to not make every kill a particle fest, but what if it was just first bloods that got to keep it?
(Note: Riot, if you do this, I will buy one.)
EDIT: as suggested below, also having this apply for pentakills would be similarly awesome. Honestly, it's even more fitting on pentakills than first blood, so if it were just on that, I'd be totally cool with it.
EDIT 2: Perhaps in the future, there could just be a new purchasable category in the store called Finishers. Finishers would be particle effects that would apply only on things like First Blood, Ace, and Pentakill. They'd be bought for RP. Anyone that already owns an URF Finisher icon at the time of implementation would be gifted that finisher that they could equip as if it were a ward skin. This would be the true ideal as I prefer my Shurima icon, but I *really* want a space laser if I get a pentakill. This would after all be a revenue source and I'm sure people would pay 250 RP for it.
EDIT 3: for anyone who isn't aware for whatever reason, here are the URF finishers: