With so much great added content we just discuss whatever a player may want to use. We would probably allow anything from Town Cryer. We stay within a lore reasoning and follow existing rules where we can. No elves hired
for Dwarf Treasure Hunters, no magic-users for the Witch Hunters, etc. No one has wanted anything that we feel is overpowered.
We also encourage players to have a back-story for their warband and hired
swords. In our campaigns we normally see Ogre Bodyguards, Pit Fighters, Elf Rangers, Dwarf Slayers. All from base Mordheim rules. Then a Witch (as a dwarf alchemist) and a Bounty Hunter being prepared. These are from the Fortune Hunters article, not sure if this is official but I think so. Looking at Warrior Priest of Sigmar too from TC.
Haven't seen anything we have felt needed to be outright banned yet. The Ogre and Pit Fighter that is always hired
by our Middy player always dominate, if I could only convince them to ban those...