Nicely done XD
Give him hell Tanwen!!
I've seen such a mod: skyrim.nexusmods.com/mods/4352…
I find it totally awesome to be able to have Miraak the first Dragonborn fighting at my side while having his actual voice and abilities/shouts/spells! And this mod's also very lore-friendly, since, to have him be a follower, you have to ''convince'' him to be your ally in the fight against him with the Bend Will shout! And then the first and the last Dragonborns must fight side by side against an army of a very angry Hermaeus Mora's pawns! I find it quite epic! ^^
You can have him as a follower even if you already did the quest too from what I know, there's supposed to be a ''Skip'' option But it's true it's so much more fun to do the alternate quest ending ^^
Just be careful if you use the Dragon Aspect shout with 2 or more words of power... The Ancient Dragonborn it may summon when you're low on health could accidentally take out Miraak before you succeed in making him join you... (Happened to me once... )
Also, the fight against Hermaeus Mora's pawns is really tough... Better have a lot of health potions and be quick to use them