I use the Ancient Companion/Masters/Mystara Domain rules, with the Dragon articles and a bit of the old Castle Guide, Birthright and 3E's Stronghold Builders Guide.
I use the Ancient 2E XP mixed with Mystara XP rules, so a PC can get xp for doing lots of actions in game. And they get the most actions by each character class.
I use the Knockdown Dice idea from 2E for spells as I like the idea of blasting spells moving targets around.
Karma and Power Stunts from the Old Marvel game.
"Shields shall be splintered" from OSR