Some ideas and suggestions for putting captured illager prisoners to work making illager-powered farms, and also how to capture them because some of them can be quite dangerous.

Vindicator

As you might know, a vindicator named Johnny will attack anything and everything (that isn't an illager). This, of course, means we can create mob farms using a Johnny vindicator.

For example, in the screenshot above I have one such vindicator in a chicken farm. A group of chickens lay eggs at the top into a hopper, which leads into a dispenser that automatically shoots the eggs, creating baby chickens. When I'm feeling hungry I can push a button and drop all the chickens through a trapdoor in the ceiling into Johnny's room, and he'll do all the butchering. The hoppers underneath the carpet will pick up the meat and feathers and put them in the double chest. Place some vines in the block that the chickens occupy so that they won't take entity cramming damage.

Pros:
- It's fun to watch
- The vindicator's axe won't break
- This killing chamber can be added to any existing farm design

Cons:
- Vindicators can't pick up tools so you can't give him, say, a diamond axe to speed up the butchering, but he can usually one-shot passive mobs so this isn't really too big of an issue.
- The chicken will be raw
- Not the most efficient design (but honestly you'd build this for fun, not for efficiency)

Variations:
- Try different passive mobs, although chickens are still the easiest as they provide their own (spawn) eggs
- You could also try hostile mobs but they will attack your vindicator, so remember to heal him up from time to time (lest he dies)

Pillagers

Ah, the pillagers, I love pillagers. If you put a boat down in front of one, he'll get in, and if you jump into the same boat as him, he can't actually hit you. Although he'll still shoot arrows, the arrows will fly straight through you, and when his crossbow runs out of durability it will break (which takes around 16 minutes; in the meantime you could perhaps take him sailing).
After that, he'll become passive and you can do whatever you want with him, like building a pillager village or just putting him into an iron farm.

Pros:
- Who doesn't want a friendly pillager?
- Can be put into an iron farm, to scare the villagers without the possibility of the villagers dying (which might happen if you use a pillager with a crossbow)
- Place him in your village (perhaps in a boat so he won't run away, or in a sealed house) so that if you accidentally walk into your village with Bad Omen, a raid will trigger without any raiders spawning since the game notices there's already a pillager in the village. This is a good way to protect your village against accidental raids. Do be sure to protect your pillager from iron golems though.

Cons:
- The game still registers it as a hostile mob, even though it won't attack without a crossbow. This means iron golems will still attack it, and it will despawn on peaceful difficulty.
- Once you get 128 blocks away from him, he will despawn so remember to nametag or put him in a boat/minecart.

Evokers
There really isn't actually anything fun you could do with evokers, maybe you could make a sheep colour changing farm (from blue to red)? Honestly, it's probably better if you killed them for their totems, because they (and their vexes) can be quite dangerous.

Ravagers
The youtuber Pixlriffs had the idea of using ravagers to farm crops, since they destroy all crops and leaves that they touch. Now I haven't watched that video yet, so I don't know how he did it, but in theory you could put farmland on either side of a railway and use powered rails to move a ravager through to collect all the crops once they grow. You could then use hoppers or a hopper minecart to collect all the drops. Since a ravager's hitbox is very big, remember to leave one block of space on each side of the rail (or use glass) so it doesn't suffocate. This is also where you can put the farmland.

Pros:
- It's epic
- Ravagers in minecarts look ridiculous
- You get bragging rights for turning a ravager into a lawnmower

Cons:
- It's rather difficult to capture a ravager

How to capture illagers
This guide would be totally useless if I didn't add a section on how to capture the illagers in the first place.

Woodland Mansions:
If you're taking a vindicator from a mansion, you could pillar up three blocks when a vindicator chases you, and quickly place some blocks around him so he's trapped.

Pillagers:
It's easier to catch one in a patrol than from an outpost, since there are less of them. First, separate one from the group (by leading him away or killing the others), put down a boat and get into it as well. Then make a (slow) getaway by rowing the boat on land. Try not to leave the boat, otherwise you will get shot. Again, they can't hit you if you're in the same boat.

Raids:
I also got this idea from Pixlriffs: take a villager and his bed to the middle of nowhere (but preferably somewhat near an outpost so you can get Bad Omen quickly), protect him, put rail circles around the area, which will both stop the illagers (they can't pathfind over rails) and allow you to pick them up by putting lots of moving minecarts on the (powered) rails.
You could set up a tripwire system to send off occupied minecarts to a collection point, and to separate the ravagers from the other illagers.

Place a tripwire one block above the ground, a few blocks before an intersection. This will allow unoccupied minecarts to pass through, while occupied minecarts will trigger the tripwire hooks, sending off a redstone signal which you could use to toggle the rail at the intersection, sending off the occupied minecarts out of the loop while still keeping the empty minecarts in the system.

One flaw in this design is that stray passive mobs can also get into the minecarts, but they shouldn't cause much trouble because you can just kill them.

To separate the ravagers, use this setup: (ignore that redstone torch, it's just to power the rails)

The ravager's hitbox is wider than one block so it triggers the tripwire even though it's still in the minecart. Use this redstone signal to toggle a rail at an intersection. You must build it out of glass, otherwise the poor ravager will take suffocation damage when passing through. The repeater (to the left of the glass block) is to add a short delay, otherwise the rail toggles too early. You could experiment with the delay length depending on how far away your intersection is from the tripwire hooks, but four ticks works fine for this setup here.

Once you have your illagers inside minecarts, you can use a minecart with furnace to push them to wherever you want them. This is preferable to just pushing them yourself, because they can still attack while in minecarts. A furnace minecart can also push ravagers so you don't need to worry about wasting shields, getting hurt, or the ravager getting stuck on the rail somewhere.

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Thank you for reading this, I wrote these notes for future reference, and I hope they helped you too. Have fun making your own illager-powered farms.

Sources:
- Minecraft Gamepedia
- Own testing

[​EDIT]
Here's a working ravager farm as an example- to prove that it's possible to build in survival without getting headbutted to death. It's not automatic, nor the most efficient, but it's the most fun, in my opinion. I managed to get a ravager with a pillager rider. The way I got his crossbow to break was holding up shields to deflect the arrows (it took me about 4.5 shields because I was playing on Hard; they deal less damage on easier difficulties so you'll use up fewer shields in Easy).

Now presenting, my brand new ride-on CropMower 2000 (and yes, that mansion's my base. I thought it fitting to put illagers back into the mansion after I defeated it)