Auras normally has a radius limitation. In a tactical battlefield, an aura can span several spaces and any unit within the space will gain the bonus. As a hero raises in ranks/level, the radius of the aura could expand and/or increase the bonus.

This is could be a good play balance as players who would want to gain the bonus, need to move in a tighter formation. So this players could either move with other units with the added bonus or opt for another strategy thus sacrificing the bonus.

Similary, heroes could also have a negative bonus that effects enemiy units that falls under the rule as mentioned above.

I thought this thread was about spells like Protection from Evil, 10' Radius, but now I see y'all are talking more about an ability for hero units.

For things with specific areas-of-effect that we'd need to manage on the tactical map, I'd prefer to see that left to spells & magic items. But it seems like the basic stats for a hero could easily include things like morale bonuses for the unit/stack that it commands.