Long War Studios, developers behind the most famous (and quite frankly the best) XCOM: Enemy Unknown mod Long War, have already released a whole slew of high quality mods for XCOM 2, and they are showing no signs of slowing down.
If anything, they are actually speeding up given that their latest "Long War Alien Pack" mod is filled to the brim with a whole bunch of new and unique enemies to fight, some of which are improved versions of the previously released boss aliens. Here's a list of every new enemy you'll be able to encounter:
* Muton Centurion, a Muton leader with the ability to inspire its allies
* Muton Elite, a powerful Muton who can dominate in battle
* ADVENT Drone, a small, nimble crowd-control robot that hunts for hidden enemies
* Naja, an alien sharpshooter related to the Viper
* Sidewinder, a speedy alien skirmisher related to the Viper
* Sectoid Commander, a stronger sectoid with area-of-effect abilities
* ADVENT MEC Archer, a mechanized unit built to bombard at long ranges
* Chryssalid Soldier, a larger, strong Chryssalid
* Hive Queen, a monstrously large Chryssalid
* ADVENT Sentry, a soldier specializing in overwatch abilities
* ADVENT Grenadier, a soldier with a grenade launcher
* ADVENT Rocketeer, a soldier with a rocket launcher
* ADVENT Gunner, a suppression specialist
* Many of these have upgraded versions that appear later in the campaign, and the mod includes upgraded Vipers and Archons as well.
On the other hand, if you just want to use the old Muton Centurion mod that adds a nasty boss for you to fight, without any of these new and potentially imbalanced additions, you can find it over at the Nexus. The pack you're looking for is labeled as Muton Centurion Pack, so don't download the latest version as you'll get all of the previously mentioned enemies as well.
Have fun!