Level size felt fine.
The blood felt very feast or famine. I like the blood pressure, I'd recommend increasing the background decay rate and increasing the amount received from bloodpacks because both of those felt unnoticeable. The rates given for the horde and bat mode are good.
Speed boost could be nice, temporary immunity to blood damage might be good, some mode where you can eat the enemies for massive blood boost would be fun.
Movespeed of the horde felt good, pressing but not unfair.
With how minimal the mechanics are you could maybe do stuff like distance run, blood drank, blood lost, time spent as bat, etc. It would be a fun novelty to help distinguish runs.
Not really, some variety of environment or enemy types may help. As it stands it can feel very repetitive and one run feels indistinguishable from anote
The idea is interesting, however if that is the plan concerns about blood have to become more pressing. As it stands I feel like there's more than enough blood meter at the moment. So maybe start off with a smaller meter that gets bigger over time or make blood drain much faster so it feels like a worthwhile buff.
Other Notes: Only major issue is with the obstacles, the color pallet is very dull in this game and that's fine but it means that obstacles are hard to notice and it is not amusing when I die because I got stuck on something I can't see. In addition the visuals scream 2.5 D but the game plays like a 2D side scroller which lead to strange issues with hitboxes when my head would pump against a bush I'd argue is above my current position.
Overall a solid game, the music and visuals are really charming, and the core gameplay loop is solid.