I'd like make the following suggestion:
Why not have visual identifiers for killing sprees?
What do I mean? Well, for example, if you kill 3 Demigods without dying, as well as the 'killing spree' voice over, you get a permanent visual identifier on your hero - ie, a silver halo or an aura. This will help identify which players are on killing sprees, which could assist in setting up ganks and ambushes on those players.
I'd also like to see your 'value' go up if you are on a killing spree - so if you are on a Demigodlike killing spree, you should acquire a Golden halo and be worth 500g to an apponent that kills you.
There should also be a loss of gold when you die. This actually makes dying a bad thing and forces you to preserve your hero more.