Flaming pigs, Gladiators (probably a bit late for EB) and other limited use fun units could be implemented as mercenaries instead of faction units so they don't chew faction unit slots. They could then be made available only by script to certain factions or circumstances. Obviously a use of incendiary pigs would automatically increase your supply level.I would love to see peasants and stone throwing peasants in rebellious settlements as in EB1, maybe even added by script as mercenaries...
I think the M2TW vanilla has some form of wagon (IIRC it held the cross) with combatant priests/guards. Perhaps it could be modded to be used as a similar inspiring holy symbol/group of sacrificing druids for certain armies or alternatively a baggage cart or sige device for the human player.
After cataphract camels, camels disguised as elephants!