Sigh... Firstly, smoke is a skill used to make the ninja invisible for a reason: Because the concept of a ninja is, you cannot see it until it strikes, hence the essence of smoke. Removing it, then adding cobwebs would not serve any purpose but to slow the ninja down in a fight (Because... oh wild thought.... Web's slow people :O ) And make the disappearance of the ninja (9 meter diameter - can't do that - it'd have to be 4.5 radius ... math aspect aside, lets assume 4 block radius) .... Making the disappearance of the ninja... not really disappear..
Furthermore, I like the idea you have of there something showing up when a ninja smokes, not really a firework animation like energize (which is sorta unstealthy imo) but like a 3x3 area of the smoke animation. It's flexible, but simply making cobwebs for 8 seconds in a radius of 4 would not only draw attention, and slow you down ( btw, backflip, jump, and velocity skills would not work here, neither does escape artist) BUT it would also more than likely cause your vision to be disfigured, AND you don't go invisible like the skill suggests, which after being slowed, you would instantly get mutilated by the enemy(ies) because you were stuck in there for 8 seconds allowing them to surround you.
