really nothing you can do about it right now (especially if the band isnt coming in to do any other stuff)
but some harmonies would be awesome in the singing parts
and if the screamer is kinda weak (which in this case he is), try double tracking screams and getting a really tight performance. it could really fill in the weakness
to me talking about mono vocals doesnt always mean double track vox and go hard left and right but being able to tell that there is only 1 take right up the middle for the entire song. it could get a tad boring after listening to it this way 10 minutes into the album if you know what i mean.
Edit: I figured I should add before you start throwing vocal pans all over the place. Maybe when trying to do some harmonies, have the lead vocal up the middle and double track the harmony out to the sides anywhere from 25-100, but I dont really go over 75.
Just some stuff to think about and test out next time. its all a learning processes