Simply put you want to add exotic armies than recreate dogs of war with troops from these different factions and worlds. See how gamers react to them, it is better to vest in one unit of, " Cathaian dragon rocket batteries", than a whole army.
Humans at their core are all similar, we have; spears, bows/crossbows, swords, halberds, etc. Heavy cav, light cav, lords, hero's, champions.
So make all other human populations ( not old world) , one book with these core type troop, maybe one or two different troops, like Kislev wing lancers , Tallian Pikes etc. and of course cool hero characters.
The Empire has to go back to the 4th edition army selection, as it was a melting pot of the world, thus you will see a unit of dwarf expatriates, Ogres, HALFLINGS, and then go weird with the early engineering steam punk craziness, deep religious zealotry, etc.
