I do not know if this idea has been brought up, but I think the OWNED and hero kill announces should have unique sound from the champion committing the kill.
So if Katarina kills a champion, instead of the same announcer, it would be her voice with maybe her own witty line saying something. This way, it's easier to tell who got a kill.
Make the saying short and sweet in case of quick multiple kills, but it would bring something cool.
Welcome to summoner's rift guy has to go =p, though he sounds like Smite from WoW, it still feels out of place. Maybe have different themes for different maps and have an announcer who matches the theme.