Local co-op with other players online
At the moment you can only play with players online OR you play coop on the same computer but you can't play with one friend at your computer while having two other friends join you from elsewhere. A game where this is well implemented would be Curve Fever.
Split-screen for local coop
It's a bit annoying that you have to be near each other in local co-op. You can't, for example, clear one passage while the other player deals with the other side. A split-screen would solve that.
Synchronisation with the Steam Cloud
Not much to say about this one. It's not necessary because saves in Hammerwatch are not that important but it would be nice to have it, in case the hard drive crashes.
If these features would be implemented the game would be perfect. Great job so far, I really like the new campaign.