So, can you then turn any defeated enemy into a Thrall? Once there, can you replace with another? How does the process work? Obviously, you'd want the highest level Thralls possible...
In the Vanilla game, you can re-animate enemies that are up to Level 40. However, there are certain things that you can do to make them into tanks: you can improve and enchant their armor, but it *must* be the exact same armor you've taken off of them, improved, and put back on them. For instance, let's say I kill a Thalmor Justiciar, who's wearing a full set of Glass armor. I can take the armor that he's wearing, improve and enchant it, and then put it back on the dead body and then Dead Thrall it: he'll then wear that armor.
However, if I've created a set of Glass Armor that I've personally made and already enchanted and improved, he'll never ever equip it, for some reason. No idea why it works like that, but it does.
Also, Dead Thralls and Followers don't receive the benefits of enchantments the same way that the Player does; some work, and most don't. There's a lot of debate as to which actually work, and which don't. Any "Fortify Health" perks seem to work, but anything that improves a Skill (like "Fortify One Handed") don't. I'm still waiting to see a definitive list, and haven't been able to find one after scouring teh interwebs quite thoroughly.
Dead Thralls will equip ANY weapons and armor that you give to them, although if you give them multiple weapons, they don't always choose the most powerful...that's why it's best to only equip them with one Melee weapon, one Bow, and one Staff at the maximum.
Dead Thralls have an unlimited weight-carrying capacity, which means that you can use them as unlimited pack-mules. You kill them, put all of your unneeded crap that you're going to sell on them, and then re-animate them; they never turn to ash, like other enchantments will do to re-animated bodies. However, if you decapitate them, you're screwed - they can't be reanimated. In any case, the "pack-mule" aspect of having a Dead Thrall is awesome, since you never have to worry about becoming over-burdened, but let's say you've given them an improved and enchanted Legendary Daedric Sword, and then load them up with a bunch of stuff that you want them to carry, including a bunch of unimproved Steel swords - it's quite possible that they'll use the unimproved Steel Sword as their primary weapon, for some reason.