My thoughts and Ideas

Firstly, I think versus is alot of fun and a good starting place.
Realizing this is an alpha and there is alot of room to add, here are some suggestions:

Gas rat: in main game gas rat has a sacrifice ability and can run at enemies and explode. that could be a cool tactic to split heroes and give the class another dimension.

Assassin: could use a landing spot target similar to slayer leap when targeting the ground.

Hook rat: an indicator similar to trueshot volley to know that you are locked on, would be especially nice for people playing on higher ping.

Gun rat: movement speed could be a bit higher while you are revving up the gun, not while shooting it, just while it is preparing, to more easily peak behind corners, you are already a large target and easy enough to kill so I dont think this advantage would be too much.

Fire rat: a tad more health would be nice, considering gas rat can do more damage from a safe distance.

The speed running of heroes is a huge problem, the npc rat horde is both too weak and not enough numbers, the heroes can basically ignore them 95% of the time.
I had an idea about this and there is a couple ways you could implement it. You have the tech from twitch mode which allows enemy spawns and also buffs to the heroes, You could put it in without the twitch voting aspect(or with it, that would be cool) except each vote would spawn in different types of horde and elites but also buff the team of heroes.

or you could give limited abilities to the skaven team that spawns horde in similar to necromancer sienna in a certain place to give player agency. Basically things that will slow down or split up the hero party.

I also think that the hero party shouldnt get health draughts when they start, they should be placed in different locations around the map to force the team to have to make decisions. The heroes have potions, bombs, full ammo and full health and ults when they start, that is plenty to be able to make it fair.