A bloodlust mechanic was the first thing I thought of, yeah lol. It would definitely help to incentivise seeking out enemies
But, I'd say the first real issue that needs to be tackled is the general level design and how zombies can't really get you on cars and high parts of the level like that. All the incentives in the world won't really help much if a player can mostly just camp and the zombies keep coming
And yeah, I'd say a flying enemy type could definitely help in this regard ( or at the very least, an enemy type that can get you no matter where you are in the level ).
Though I definitely understand not making more than 1 enemy type in the timeframe of a jam. Even I had ideas for 2 more enemy types for my game but had no time to make them lmao.
Colour tinting enemies does work as a good way to differentiate them though. Definitely something to keep in mind.