An easy, elegant fix to make the other mercs more viable and open up way more fun build combinations. These would all fit thematically very well, too.
Rogues
|Blessed Aim aura.|
-This naturally fits thematically, and opens up a great niche for physical fighters especially in the early game. Between inner sight and blessed aim, you’ll get a great bonus to your ability to land blows on enemies. Freezing arrow is a nice CC option, too.
Iron Wolves
|Holy Elements auras.| (Holy Shock, Fire, Freeze).
-Again, a very natural fit. This bumps up their AOE damage which is lacking, further solidifies them as the master of their element, and for the Ice merc, instantly makes him viable due to the excellent utility holy freeze brings. Static Field and Enchant were excellent first steps, these auras propel them into viable options.
Barbarians
|Vigor Aura.| Also, as a general change, let curses stack with other classes of debuff, such as Battle Cry.
-Between these two changes, you get unique utility from the Barbarian, and by letting curses stack with Battle Cry, there are no reservations about giving him a Lawbringer to use, for example.
With the above changes, you provide a ton of great utility to the other mercs and make them attractive options. The act 2 merc still remains extremely powerful with the Might, Defiance etc. auras and itemization potential (Infinity, Pride, Reaper’s Toll, etc.)
This is not a cop out nor a lazy idea - Act 2 Mercs are not Paladins and thus it doesn’t make more sense for them to specifically have auras and not the other mercs. Arguably, Blessed aim on a Rogue makes 100x more sense than it being on a Desert Mercenary.