Militarists have the ability to create armies out of city defense units. I think having the Zulus be an army primarily focused around the militarist ability to levy troops (like the huns or mongols but more balanced) would be very interesting and also historically accurate, as their army was mostly fielded from humble citizens in a need to defend their homeland from the English. Impi would also be what attacking armies would have to fight when sieging a city, unlike mongols or huns who still use generic militia.

With this idea in mind, them being a free militia unit means a few things:

1. No upkeep. perfect as Militarist civs need all the help they can get, and the proposed 12 money upkeep and such a unit isn't doing them any favours

2. Free healing to max HP. Being militia means they have no production cost and thus no cost to heal, essentially meaning any Impi left alive can return to full HP so long as they reach their home territory. ( I think this can be incredibly fun and wonderfully broken depending on what stats the Impi have which I will get to later)

2a. Their current legacy bonus is useless for them as is. To be fair their legacy bonus isn't that good, by this point in the game units can healed without much thought for the money cost, and I imagine the problem only gets worse in the next and last era. A better legacy trait that would have impact at this point in the game would be the 50% regeneration in ALL territories, so that momentum can be maintained during a war without having to pull back and heal troops. If not even that is a good enough legacy trait I'd throw in "units immediately regenerate after combat" which means 50% of that HP or more is regenerated the turn a unit ends fighting, rather than having to wait one. Again this may sound broken but this is the second last era of the game, militarists need some kind of bonus to stay competitive that isn't +5 ransacking strength or +20 fortification on garrisons only.

3. Can move the turn they're spawned. Militia units can move the same turn they can spawn, and I've incorporated that into their new unit stats, since it allows for a lot of flexibility.

4. The militia tech of the industrial era unlocks later than earlier eras. In most eras the new militia upgrade is one of the first things you can get, fortifications, feudalism, the thing that unlocks draftees I forget. However in the Industrial era the new militia unit unlocks with nationalism, which is actually a bit later down the tech tree. Having Impi be an immediate unlock militia unit would be even more beneficial in this era since the militia tech appears later than normal.

Alright on to the new proposed stats, keep in mind this is in addition to the new legacy trait I proposed in 2a.
Melee: Close-combat unit
Unstoppable: +3 CS when 1 tech era behind enemy, +7 CS when 2 or more tech eras behind (rather than a generic +6 when attacking, also unlike the old one this one doesn't just apply when attacking, although maybe I should keep it as only when attacking, and increase the numbers a bit as well)

Production cost: 0 (it's a militia unit)
Upkeep: 0

Strength: 41 (Same as the halberd from one era earlier)
Movement: 5 (One more than a regular infantry unit, combined with their militia status means they can traverse large amounts of land very quickly)
Range: 1
Resources; None, 1 Population required

Let me know if you think these numbers should be changed

The reason I chose the strength stats to be the way they are is to prevent players from rushing for the impi in order to conquer their opponents easily. A similar problem already exists with the mongols and huns, but those come at an era with far less population than the industrial. Lowering their base CS to be more in line with the halberd was done so that rushing ahead of other nations in when you're in the early modern era to unlock impi doesn't make your units oppressively better. Remember anyone can use any culture, and making Impi too good of a Militia unit at base CS means that a player who's ahead can use them to defeat every other player with little consequence. Having it be 41 means they at least stalemate against other Early modern cultures but still have advantages in being faster, heal instantly in their own territory, produce instantly and free. Making their combat bonus against more advanced technology on a per era basis also makes them better as a catch-up tool so that players that are only 1 era ahead aren't taking as much damage as those 2 eras ahead.

Just going to toss it out there, but the industrial era militia (conscripts) are just better then what has been suggested here.

Conscripts (seen in Victor) are 43 Str. gunners with 4 range, 4 movement, and 0 upkeep.

As typical for militia units their str. is low for the era, but attacking without incurring a counter attack is a huge step up from the Early Modern Drifters. Being a home guard unit, at least 5+ of these will spawn in sieges and will receive defensive benefits (Fortification +6 str & Guardian +3 str.). With the purposed LT change, they would be even stronger defensively or offensively then making Impi an EU militia unit. These unlocked at Nationhood which is one technology removed from the starting industrial tech (right after Encyclopedia).  

Interestingly conscripts stack up very competitively against musketeers by exchanging 3 str. for 0 industry cost, 0 upkeep, available on-demand (militarist ability), and able to be transferred quickly between cities without other modes of transportation. To transport these quickly... have them enter another city's territory -> disband them -> re-levy them and continue onto the next city's territory to be repeated. Using this method... these can cover tons of ground with minimal technology. 

RNGZero wrote:

Interestingly conscripts stack up very competitively against musketeers by exchanging 3 str. for 0 industry cost, 0 upkeep, available on-demand (militarist ability), and able to be transferred quickly between cities without other modes of transportation. To transport these quickly... have them enter another city's territory -> disband them -> re-levy them and continue onto the next city's territory to be repeated. Using this method... these can cover tons of ground with minimal technology. 

What the hell? that's crazy. God not even the regular impi would beat being able to spawn a ton of conscripts as the zulu.

@RNGZero on second thought do you think it'd be better if we keep the stats I came up with, precisely because a militarist faction that could spawn armies of conscripts would be too powerful? Although chances are that Zulu players wouldn't be close to unlocking conscripts tech-wise. Then that leaves the Germans. Honestly I forgot they were militarist for a second... Oh boy that could lead to some problems in the future.