Yes I agree with the CP. I was trying to come up with a “unique” spin on it that would fit nicely in classic without causing to many balancing issues. Rogues would probably find a way to totally break the game if they got CPs on themselves instead of the target 
At the very least they should keep CPs when they tab target as long as they don’t use any CP builders on another enemy.
Of course any CP changes would also affect druids.
I don’t know about Fan of Knives though. Feels like a too easy solution. It would work but be kinda dull.
Another alternative to my Lava Lash redesign is to just slap a “when used with Flametongue increases all fire damage done by 20% for 10 seconds” and call it a day. Dull as hell but it would kinda do the job.