mercenaries are not that useful in most end-game content. they die to almost any end-game boss aoe ability and most melee attacks from inferno zones. most people use a3 merc because they have slightly higher chance to survive due not being targetted a lot.
i was thinking of a couple of ideas and this is what i came up with, for tanking part:
- mercenaries get a neutral resistance and max resistance boost (+50 AR, magic res and %10 max res), gets passive +10 avoid, dodge, evade and some life per hit. I guess bonusses may vary from merc types. from strongest to weakest: a5>a2>a3>a1. i still don't think any of these will make them sustainable in end-game contents including bosses.
- a mercenary specific item (like a charm) that makes them invulnerable. this is in diablo 3 and hurt no one. would be a world drop grand charm.
also their skills and offensive usefulness are very limited. my recommendations for each type would be:
act 1: specific elemental arrow, decoy, inner sight, slow missiles and multishot. may cast lower resist on striking.
act 2: fanaticism aura (offensive), vigor and blessed aim aura (combat), salvation and cleansing aura (defensive)
blessed aim aura can get a life per hit bonus maybe.
act 3: lightning: chain lightning, enchant (with full lightning synergy), static field, lightning mastery
cold: blizzard, enchant (with full cold synergy), shiver armor (can be used on allies), cold mastery
fire: meteor, enchant (with full fire synergy), fire golem (with conviction aura), fire mastery
act 5: bash, seismic leap, heart of the wolverine aura. may cast bloodlust and acid injection on striking.
i think these changes would make mercenaries an exciting way to build your character, since they'll benefit much more. right now they are aura carriers until they die, then maybe you revive them or not, %95 of the population doesn't bother reviving them.
