You have in fact several human armies in KoW, and most of them will be able to represent some aspects of the Empire.
I will first mention the Brotherhood and put it aside as the equivalent to our Bretonnian neighbours.
The other human armies are :
- Kingdoms of Men : general humans with few exotic options but lots of mundane ones, perfect for example for an Altdorf army, with maybe some wizard support from the colleges of magic and one general on imperial griffon if you still want something slightly exotic. They are the vanilla version of the Empire
- Basilea : perfect if you want a Sigmarite army with warrior priests, flagellants, and so on. They can also allow you to use the Stormcast Eternals if you want (the winged ones as Elohis, the ones on foot as Ogre Palace Guard)
- League of Rhordia : they are the more exotic options of the old Empire armies, you still have normal human soldiers, including some elite dogs of war, but you also have options for several themes : you have options for the demigriffons (and a griffon like hero) if you want to make some monstruous beast knight order, you have exotic war machines to represent fairly well a Nuln army (including a steam tank), and of course you have halflings !
There is some overlap between the three armies, but depending on your preferences you can use them to build many varied armies, and you can also combine them by using allies if you want a different mix.
Also remember that most magic items can be equipped on normal units, not just heroes, and this can be used to transform an unit into something else. For example the war bow or the flying hammer are cheap artefacts giving a lone ranged attack that can represent things like a witch hunter with a pistol or rifle in a middle of the unit (or alone if you give them to a normal hero)
The diadem of dragon kin give a breath attack and can represent many things, from a fire wizard , to an engineer (I have a kitbashed engineer model with a hand grenade launcher that I sometimes put in an infantry unit to represent the diadem breath attack of the holy grenades magic item)
Also let's note that with KoW 3rd edition just about to be released we might get new options.