zeds? blood plague? freaks? nah... you know, I think there is something even more dangerous than a simple bio-weapon walking on the surface of earth, and that's the minds and hands that made it possible in the first place.
now, so far, every encounter with a hostile survivor group ends up either with a pretty short battle, with the player taking advantage of their dumb AI, getting them headshoted pretty easely or backstabbed while they're fighting with one of the undead attracted by the noise of their gunshots orrrr... a hellish and almost unfair battle in which a headshot doesn't even count as a headshot to them and not only that, but they are also some kind of god-like expert with their aiming so they get you in a matter of seconds dealing to your survivor a whole lot of damage, all of that without even caring if they are out of cover or if they will get shot by you or the ONE and only survivor you can take from your community to hunt down a whole enclave...
hell, they don't even care if the noise from their gunshots is gonna get everyone killed ( including themselves ).
(*and speaking of "dumb AI", now that I remember, you can even get them all dead on the ground just by hitting their whole body against any car a few times on any diffculty, pretty easy. )
So my point here is: the whole human AI needs a little upgrade when it comes to fighting against other humans.
I think the "division" hostile AI is the best example. and I'm not talking about the second game, you can take some good "tips and tricks" from the first one if you don't want to go that far...
enemies quickly take cover even if they're not getting shot at, take a peak through their cover, if the players gets too close, goes full agro while serching for a new cover. And from the other hand you got the ones that run directly to the player to get him out of cover.
I guess you guys can go pretty crative with this, maybe making different AI for the hostiles, so you've got the ones that are pretty cautious and stay in cover and the ones that are constantly trying to get you overrun. or maybe a rotative AI that does any of those two when needed. And of course, perhaps you could make them "flee" if they're too injured or if freaks like bloodplague jugs show up right in the middle of the battle (and/or making them use their firepower only if you rise up yours, otherwise they'll go melee or just point at your survivors with their guns, so they don't get the attention of the whole undead army that could easily overrun their whole community and base).
but of course, all what I've said above it's just a suggestion, do what you want because I don't care how you do it, and like I've said before (aaaand a summary of this whole wall of text): I just want some enhanced hostile survivor behaviour overall. that's about it!