Here is my take for this excellent idea (except for the slowing down bit).
3 Kills: Flamberge swings thru everyone in its path.
5 Kills: Flamberge gets crush through and knockdown, being a beastly weapon.
7 Kills: Flamberge can block any incoming projectile, automatically
10 Kills: Instant round win + 10k gold for the hell of it.
15 Kills: Nuke? (Unattainable bcos of the 10 kill streak bonus anyway).