So, how many zeds are we talkin’ here? Hundreds? Thousands? Massive numbers of any type of zed, even hulks, are meaningless to a fully kitted out survivor. Heavy survivor armor for melee damage, dielectric capacitance CBM (or Faraday shark suit if you’re lucky) for electrical damage, acidproof mutation for acid damage. Take any martial art with a counter attack and watch the horde break on you like waves on rocks. You’d be in more danger of dying of dehydration than damage taken. It only gets more broken if you use the vehicle additions mod and use the AoE electric turrets.
Once the player gets powerful enough, they’d probably end up actively seeking out mega hordes due to the presumably high concentration of shockers (shockers = power storage CBMs = plutonium cells). And once they clear out all the mega hordes in the area just like they cleared out all the labs, they’ll just end up drifting aimlessly again.
Oh lord, and what about the aftermath? All those corpses to smash…
I’d say if you want a challenge as an endgame survivor, go for quality, not quantity. And “quality” does not just mean “how high can we pump up these numbers”, because in the end the survivor will always win the numbers game. Be creative. Make the player work with multiple mechanics to reach their goal, give them options, and give them incentive. Taking the labs for example, there are several ways to simply get inside (ID card, electrohack, explosives, jackhammer/pickaxe) and deal with the turrets (facetank, EMP grenades, smoke grenades, cloaking). There’s a variety of enemies (zed scientists, manhacks, robots, bio-operators), and multiple reasons to raid a lab at every level of play (chemicals, CBMs, mutagen, the finale).
Or, well, that’s just how I see it. I just love labs. If you want a massive all-out brawl, that’s cool too. But I don’t think stuffing the reality bubble to the point of popping is the way to do it.