I feel like the amount of infected in this game is a bit low. You basically unlock them all past level 25, with the HERC riot infected being the only zombie unlocked past that.
I'd like to see that change, so here's a few zombie ideas for mid to late game (along with some zombie level and stat changes)
HERC Scientists: Currently they act exactly like normal walkers/runners, so I think giving them a small boost in health and/or speed would make them a bit more unique.
Police Zombies: I'd love to see police zombies get split up in a way. Remember in say Last Stand 1 or 2 how there were basic cop zombies? It'd be nice if the current police zombies were made into "Swat Infected" while a new type called "Cop Infected" (or police, doesn't matter that much) was added. Police infected would show up at level 10, and would be slightly slower, but with a decent amount of health and a small buff to damage, while Swat would be found later on.
Majority of the current zombie types: Move them apart. Riot infected should start showing up at level 35 (maybe 40?), while the other types (military, herc) would show up in between levels 15 and 30. This would extend that period of the game where you unlock new things (level 25 is really where the game ends for me personally) and discover new enemies; 30 levels is a long way to go before you encounter a new threat in the form of HERC Riot infected.
(Yes, I’m aware that some tasks want you to kill certain zombies in certain areas. An easy fix would be to simply change the zombie type required to something else)
Construction infected: Infected in mild to heavy construction equipment. They generally have a bit extra damage resistance, more so depending on what they're wearing (construction helmet, thicker construction gear, welding masks, etc). (This is a maybe, but perhaps they sometimes wield melee weapons, like hammers?) Appear at level 14.
Melee infected: These infected are quite literally the exact same as their non-melee counterparts, but they gain upwards to a 50% damage increase depending on the infected type and melee weapon. Infected will have a higher chance to spawn with melee the further into the game you go, but to make this balanced, more powerful melee weapons would have a much rarer chance to spawn. Infected “unlock” more melee weapons the further you level up, but they still have the potential to spawn with lower level melee weapons. These infected do not gain any health or resistances, but simply an increase in damage. Much more common in high activity zones but can still be found randomly throughout the dead zone. They start appearing past level 20.
Survivor infected: A mixed bag of stats, these infected are survivors who were not immune to the virus, but survived long enough (until now). These infected would have similar traits to the current survivors; recon infected would be extremely fast, fighter infected would be slow and extremely tanky. These infected would be much rarer than normal infected, and would often carry melee weapons to increase their damage. These infected would almost, but not quite be as powerful as elite infected. The one thing that would make them dangerous is their potential to come in numbers. These infected would show up in high activity zones; clearly these zones are dangerous and have been noted to be so by herc, so its not that much of a stretch to say some survivors were turned while attempting to explore the area. Appear at level 27+, only in high activity areas.
Spec Ops infected: These infected wear high grade military spec ops armor and gear, and are essentially a higher tier of military infected. They have a bit extra damage as well, and a slightly higher chance to wield melee weapons. Found past level 45.
Riot Shield infected: Heavy set Riot infected wielding riot shields. Very resistant to projectiles, easier to take down with melee. Slow attack speed, but heavy damage. This means you can have a melee fighter dart behind them between attacks and get critical hits, but if they're caught, they'll take a lot of damage (compared to normal infected attacks). Start appearing at level 45. The HERC variant appears at level 52+ and has the potential to be a runner, and also gains additional health.
Exo-suit infected: These infected were tested to use prototype HERC exoskeletons. Scientists recorded their effectiveness against survivors.
Take a unique elite infected and give them steroids, and you get yourselves an Exo-suit infected. These infected cannot show up more than once in a mission, and generally serve as a sort of "boss" for special missions, such as the raid on union island. Start to appear past level 50.
Prototype Herc Infected: These infected wear prototype Herc armor, and as a result gain solid resistances and extra health. They have a base of 40% resistance to all attacks, and they may sometimes feature up to 50% projectile resistance. These infected are only seen past level 50.
This infected type would also be seen on Union Island in a living counterpart. These units would basically be upgraded versions of HERC soldiers and would wield advanced weapons such as AS12s and other weapons. They’d be more common on the compound mission but would still be found on the monument and rarely the bridge.