You can find the mods right here on the forum just scroll down the main GD forum page to see ones like Reign of Terror (D2/1 in GD), Grimarillion and Dawn of Masteries (more masteries to play around with and can increase hero/mob density in them), Nydiamar (though I’m not sure that works with v1.2) and some others in Modding Projects.
You could even try going HC classless, i.e. no masteries at all; skills come from items. Yes, it’s possible and yes, it’s been done. Therer’s even a reward for doing it if you get to the Vanguard in HC Ultimate.