Note, this is a suggestion and is entirely opinionated.
My suggestion is to change ISF to not be so short term; I personally feel Destruction relies too much on short term buffs/spells.
Improved Soul Fire:
Your Soul Fire spell increases the damage of your destruction spells by 8% for 45 seconds and your Soul Fire spell is now instant cast. Soul Fire now has a 45second cooldown.
The cooldown is only when taking the talent, so any destruction Warlock would be forced into taking the talent and I personally don't see a downside to taking it.
Soulburn: Soul Fire
Increases the damage of your next Soul Fire by 30% (Just a random number I pulled out of my ass - something to make us Soulburn Soul Fire occasionally during a fight (Would be nice to finish off that low add or something)
Empowered Imp:
Your Imps attacks have a 15% chance when hitting a target below 25% HP to reset the cooldown of your Soul Fire and make your next Soul Fire cast as if it was under the effect of Soul Burn.
So generally opinions on this? I feel it address two issues;
1. The lack of execution damage for Destruction
2. The heavy reliability on several short term buffs/abilities as Destruction.
Obviously numbers are just numbers I pulled out my ass, I'm interested in opinions more on the concept of those changes, rather then the numbers I mention.