So far on the list:
Enforcers.
I really hope so. Even just a compilation would do. Lore wise however with the Succession arc the jackbooted thugs have taken a real battering so it may be harder to squeeze them in
Corpse Grinders have barely been touched since initial release.
For good reason. Open cannibals be Gross. The Hungry Hungry Hippos still hold up well in game play (just by being over powerful in melee) so corporate GW does not need to whip the horse anymore. They could however be parceled into a Cultists book as they are in essence a Khorne Cult.
The malstrains have their book so the "proper" Genestealers will have to look elsewhere. Like CGC I think a revamped Cult book is the way to go. I can't see a new model angle for them to plug though as they have a sufficiently rounded W40k range of to use already.
Chaos is pretty much ignored.
Ignore Chaos at your peril!
Yes they really really need an update. They have many regular human cultist sculpts across various games so the model scope is there. However it could be in danger of going full deamon or Chaos space marine which would ruin the Munda vibe. A cultist book for all three please.
Venator Gangs are dead. Outcasts just do the job better. They could do a decent revamp of the bounty hunters mechanics to get them in or like the Noble, guild, criminal and Ferryman delegation use that mechanism to keep the Venator idea alive
Ash Wastes Nomads are still relatively fresh but as Ash Wastes as an event has passed… who knows what will happen.
They hint mega buf transports, Ash lions and more so I hope so.
However they put a lot of key bits of lore around a undercurrent of a Nomads plot line in the Succession arc and left it in a big cliffhanger so I think it will be in a similar vein to that rather than a stand alone book.
Also think the Nomads muscle out the idea of Ratskins as a gang.
The lovable Hug monsters of the Slave Ogryns

.
Personally I think they could just use a little expansion on weapon selection. You would think they can handle a Heavy Maul. It's a club with a nail in it.
Also I think they should be able to use the Medical Escort actions to remove or add Augmetic weapons.
More model kits would be nice.
(Ok I accept my Idea of a Ogryn duelist with giant ass Stiletto swords is a little silly)
I don't think Pit Slaves will happen. Functionally Goliaths, outcasts Esher or corpse grinders can do the various Gladiator Combat bits rule wise and you can just use those rules with other models. It will not be a new thing for GW to sell.
A Brat gang without having to surgically reshape the nobles limbs is an idea they could get mileage with. Venetian carnival mask style.