I feel like Undead and Greenskins could both be in the future of Vermintide. Undead could have a lot of really cool mechanics - maybe on hit giving you curse (just a little, that could decay over time), which would change up entirely how you fight them - hit-trading would almost never be a good idea. Greenskins . . . well, all I need to say there is Squigs and WAAAGH! Holy crap, hearing the WAAAGH! to signal a horde would be amazing. 
Beyond that, I would like to see the heroes travel a bit. We shouldnât forget the Empire setting entirely, but the game has a huge appeal in actually feeling like an adventure. Letâs go down to the Southlands and see Karak Zorn and a Lizardmen Temple-City! Everyone knows Iâm the resident Skink-monger, but even if thereâs never a playable one, I would at least like to see a Temple City in the game. Given that most DLCs have been about âgo get this macguffinâ, it wouldnât be hard to explain. âUse our magic portal to go get a macguffin thatâs far awayâ is actually less weird than âgo into a deadly illusion of the past to get a macguffinâ, IMO.
That said, new heroes are something I really, really want to see. Not a lot; I know theyâre a lot of work, and it would compound on itself in a way. Of course, so does adding anything new that requires voice work. The absolute max Iâd think could work could be three; and if that were the case, Iâd want to see:
A Bretonnian - Could have Knight Errant, Herrimault, and Lost Son
Probably the most obvious, but the least interesting to anyone who isnât a Bret fan already - not because thereâs anything wrong with the idea, just that it doesnât necessarily stand out in comparison to the existing careers. We have a knight and a Robin Hood-esque character already. Lost Son could at least stand out as visually more mystical, and maybe use spells.
A Skink - Could have Cohortian, Mist Runner, and Astromancer (I love this idea, talk more about it here)
A stretch in distance only, as Skinks can learn a lot of languages and are totally anathema to Chaos and Skaven. After a tense intro to the party (though I expect Kerillian probably would know what Lizardmen are about, Lohner too), they could be accepted easily and be the exotic member of the party. Astromancer lets us get other cool elemental spells, like wind and electricity and ice.
A Vampire - Not sure, lots of potential from the various bloodlines.
Hardest to justify in lore - I donât think Saltzpyre would ever accept one, and literally everyone in the party has a lot of reason even beyond the obvious to hate a Vampire (Dawi will not tolerate them, nor Asrai, plus Kruberâs history with Necromancers, and Siennaâs likely to hate Dark Magic users for giving all magic users a bad name). But it could be really damn cool. I imagine them having only half of a âgreenâ health bar at max, with the rest only being able to be filled via temp health from draining enemies or something. Iâd love to see a monstrous Strigoi that could use giant claws to shred enemies.
Beyond that . . . I donât want a reinvention of the wheel. Iâd like to see more weapons, more enemies, more maps, more pickups.