Whilst I think the game is brilliantly balanced and I'm sure these could be tweaked so as not to make them overpowered:
I'd like to see a Custom Class where in the Loadout screen you pick the 1 - 4 levels from the other classes.
** Possible spoiler below if you haven't finished Mayday **
The other class I would like to see and would fit into the story could be something to do with psionics, given what has happened to one of the characters in the story. This could then be one of the 5 unlocks left in the armory, the other unlocks could also relate to this.