I dunno, I find i use build up and aim at the start of the fights but once the rotation is going i dont really use them anymore. So build up aim as useful as they are become more of a situational power. A few powersets already have some other mechanic built in that renders buildup redundant. As for your Iaijutu idea i think ninja training > Shinobi power for blasters is what you are describing. I do like the idea of making BU/A powers more efficient. so heres my suggestions
Blasters/Defender/Corrupters
Replace Build up with: Accrue Velocity - Toggle cost .56 | Ranged attacks increase in damage as they travel, 10% melee range / 20% mid range / 30% long range.
Replace Aim with : Sure Shot - Toggle cost .56 | Attack Accuracy increases based on proximity 30% melee range / 20% mid range / 30% long range.
When Velocity and sure shot are used together their effects are cancelled and a unique ability is applied
: Juxtapose - Toggle Cost .25 | Increase damage/accuracy of all attack by 20% from all distances.
Stalkers:
Replace Build up with: Double cross - Passive skill | 100% chance to perform crit when attacking from the back.
Scrappers:
Replace Build up with: Fearless - passive skill | When facing an opponent head on +15% chance to crit (In addition to normal chance to crit, Aoes are unaffected by this power)
Brutes:
Replace Build up with: Ire - Passive skill | When taking damage 5% chance to recharge a random power. (not affected by dots also does not affect t9's)
Tanks:
Replace Build up with: Deflecting Flex - Toggle .56 | Returns 3% of damage taken as smashing damage