One of my favourite things about Diablo 3 is the boss modifiers and how fun they are. Molten, arcane and frozen are personal design favourites. On the other hand Reflects Damage and Extra Health are yawnworthy. Naturally I have ideas for more modifiers.
Things that suck about Reflects Damage include: no visual effect and a lack of interesting decisions when dealing with it. Iron Maiden was my attempt to fix this.
The monster will freeze in place for a couple of seconds (after a short spell FX) and sprout rusty spikes. While frozen it's immune to damage and it responds to attacks with volleys of very damaging spikes. This gives players something to react to and something that can be played around.
I like the metallic theme and naming it after the Diablo 2 curse, but you could also make it frost-themed as a reference to Chilling Armour.
Arcane Enchanted is probably my favourite modifier designwise so I wanted to come up with something similar. Corrosive monsters will lob blobs of acid that slowly expand into large puddles of continuous poison damage before evaporating. This limits player movement in a similar way to arcane sentries.
My fix for Extra Health is to make it more interesting than a passive 50% hp boost. When a Returner monster dies, a spell FX plays over its corpse. After a few seconds it's returned to life at 50% hp. This can only happen once per monster. Similar overall effect to extra health, but more interesting to look at. And the first kill gives players a mini-victory to make the fight less of a grind.