The Enemy Randomizer Mod is a mod that does just that, randomizes enemies!
Enemies that are normally floor or room dependant can now spawn anywhere. Bosses can spawn as regular enemies, enemies can spawn as bosses, etc.

Toggeable Settings:

There are currently 3 toggeable settings that can be turned on/off using the ~ to enter commands.
1. Normal enemies can spawn as bosses. Type "rand 1 on/off"
2. Bosses can spawn as normal enemies (health modified). Type "rand 2 on/off"
3. Player starts with a random gun and item. Adds to the craziness and helps with the increased difficulty. Type "rand 3 on/off"
Enter command then restart the dungeon or enter a new floor to initialize.

Other commands lines:

• "randhelp" for help on these command lines.
• "randmodes" to get your current randomizer mode settings.
• "jank" if something didn't spawn correctly and you need to clear the room to progress.

Notice:

Instead of a boss death triggering a player teleport, bridge spawn, or other progression event, the boss death causes a procedural teleporter location to spawn in the next progression room. See Images for a visual.

• DraGun death causes a procedural teleport near the top of the room.
• Lich 1 death creates one to the right of the map, near the player entrance.
• Bullet Past death (Agumin) creates one to the right of the map.

Installing mods:
Rename file if needed to "EnemyRandomizer.zip"
To install drag the EnemyRandomizer.zip into the gungeon Mods folder. Delete "mods.txt" if it is there already in the Mods folder.
Then launch the game. If Mod the Gungeon installed correctly, you can press F1 to see the list of active mods.

Sources:

https://github.com/cspace-000/EnemyRandomizerMod

Feel free to report any bugs you find.