I think slightly improving soldier in offensive situations without making him stronger defensively is the best way to go about this.
reducing/removing fall damage for soldiers would make bombing slightly more effective and flashier plays would be slightly more viable.
I think it would be pretty cool if soldiers instantaneously loaded a rocket into their clip whenever they walk over an ammopack. For clarifications sake, they would still have the same rate of fire and they still wouldn't be able to have more than 4 rockets in their clip. It would add another element to positioning and would make ammopacks a relevant resource in the middle of a fight.
Slightly buffing the banners would be awesome, especially if it was done so in a way that makes all three of them viable without turning it into rock paper scissors. Maybe if soldier gets buffed in other ways such as reduced fall damage or whatever the banners wont even need to be buffed.