I think the issue currently im having is that there is no protection for the player-rats, as usually specials do their most damage in the middle of hordes, when they are harder to take care of immediately. In the gamemode, nearly everything on-shots you, especially ults that seek you out, and specials do nearly do damage compared to the constant heals that players get.
what if the player rats could vote on when to start an ambush or horde, with a cooldown? That way there are fewer lone rats just popping up and dying, with more mechanical teamwork support?
Maybe give Packmaster a horde spawn ability (one time use). I think the Packmaster could fit nicely into this role. Or maybe break the mold and give the player a Storm Vermin to control (Halberd Variant).
The SV(halberd) already has a stand and point emote; this could be their emote when ability is used. The ability would require LoS to give the heroes a chance to shut down the horde call.
I like this. instead of a vote, which can lead to frustration, just display a cooldown timer to show the team when the hordes are spawning.