I thought this game was great.
If you're looking for suggestions. More upgrades.
Maybe some sort of sheep swimming for water levels.
Sheep static, where there's a slim chance to infect near a sheep when they pass someone they choose to target.
Some sort of sabotage mechanism, like longer lockdown times, and incompetent police. Or some sort of media sabotage where non-defence people will willingly run into the sheep as they've been brainwashed by mass media.
Some sort of defence that is static, and isn't people, like aturrett or barricade or fences.
Maybe different types of sheep that come from different types of units, like a bizerker sheep, that gets more speed and HP when petted, or a aware or obsessive sheep, that follows you cursor to any person you click on. Or a healing sheep that goes around waking tasered sheep up.
Maybe there could be a scientist defence that will attempt to kidnap and "cure" a sheep, and requires multiple bites to get through its hazmat. A body armour police officer who again requires multiple bites, but has a melee.
A upgrade for multiple mutagen drops is definitely something that might be needed for bigger stages. Also, some sort of zoom in and out. I think the area upgrade needs to be split into area and lingering.
Also, I noticed a little bug where the army guys are often not affected by the mutagen splash, unlike the police and the general public.