The roof of the Mercy Hospital in Left 4 Dead was really cool as it was layed out like a zombie playground with all those places you could climb up to and jump down from. Even though the space was limited you never felt trapped. Not that you should copy the layout but replicating the feel of it would be great.
Also finding a way to make the inside of the hospital not be instant death for a 1 or 2 man team may be a challenge with a lot of tight spaces and running zombies. Not sure if you'd consider making only half the zombies be runners inside the hospital so that you'd have more chance to survive.
Also, I just saw a TMG video (don't give me crap) where it looks like someone ported those little monkeys from Ascension or maybe they built them from the ground up, whatever. If you could reskin those and make them look like babies that cry, maybe even a demonic type cry that would be WAY creepy. And have them come out of the NICU (Neonatal Intensive Care Unit).
Maybe there could be filing cabinets that you could move around so that once you go through a section of long hallway in the hospital that you could put the cabinet in front of it diverting the zombies from a primary path to a secondary path. If that's not possible maybe once you block their path then they stand outside the room for 30 seconds or so before bursting through, thus giving you the feeling of temporary safety and slowing the very fast gameplay a little allowing you to hit the box or buy wall weapons and catch your breath. If non of this works maybe think of someway to slow things from time to time so you aren't numb by the blistering pace and tight quarters.
A possible easter egg could be that you had to get into the elevator at ground floor and then it would stop you at each floor and you'd have to complete an objective and then get back to the elevator. The 2nd floor could be go grab a weapon or health (Jug), but then by the 10th floor you have like a whole list of things you have to do. The things is, you can go on to the next floor without accomplishing that level's task, but the more objectives that you are able to accomplish the more weapons and turrets and ammo caches will be on the roof by the time you get there for your last stand when things go nuts. Just an idea, take it, tweak it, or trash it.
I love the hospital look and feel, keep going W1NG3D!