Very very good point ! GW has huge issues with balancing, agreed (look at the rules for new Ghaz model, they suck). I think some balance is needed for sure, and rules simplicity is a great way to do this. I appreciate your explanation of why auras can be unbalanced, this is the sort of stuff that I like to learn.
HDF in GDF have ‘Battle Drills’ which is similar to what you have suggested @Darguth. This is only available on one unit/ hero.
Battle Drills: When the hero and his unit are
activated pick one of the following drills, and
they get one of these special rules until the
end of the round:
Double Time: +3” when moving
Take Aim: +1 to shooting rolls
Focus Fire: AP(+1) when shooting
Fix Bayonets: +1 to melee rolls
Perhaps this is the way forward when wanting to add more detachment rules?
Though adding in whole new special rules that apply army wide is more similar to how it is done currently for other detachments.
Knight Brothers (40k GK) all have ‘Aegis’: This unit may block spells as if it had the Psychic special rule. If it is a Psychic then it gets +1 to spell block rolls.
When taking Knight Brothers you can buy Battle Brothers and Prime Brothers units and upgrade them as shown below. Psychic units may use spells from this army instead of those from their own army.
Upgrade with Aegis:
Captain, Champion, Engineer,
Psychic, Battle Brothers, Assault
Brothers, Support Brothers,
Pathfinders, Brother Bikers,
Pathfinder Bikers, Support Bike,
Destroyers, Heavy Exo-Suits
Prime Captain, Prime Lieutenant,
Prime Ancient, Prime Psychic,
Prime Brothers, Blaster Squad,
Raider Squad, Infiltration Squad,
Elimination Squad, Aggro Squad,
Jetpack Squad, Suppression Squad
I like both ideas though, if you’re wanting to add bonus’ to hero’s, the Target X friendly units in X", this ability lasts until X is the way to go I think.