This is funny because they've already done this.
The cast time on it is about 0.2 seconds from the mage update.
The speed buff on tibbers after annie dies got lowered some patches ago.
Annie's ult would scan for hits when you pressed ult rather then when it actually appeared before the mage update. That means it went from 0 to 0.2 seconds
It's actually really annoying seeing the enemy in the indicator, ulting, then just barely whiffing it because of the delay.
When hitting it, I relied heavily on the indicator waiting to just barely get in range or when flash ulting. You could feel the change if you played her alot before/after the mage update. The change wasn't in the patch notes, but myself and annie mains could tell what happened.
That's another thing, the delay also applies to flash ult, so with a good reaction time, if you're on point and expecting annie to flash ult you, you can react to it just barely, when you couldn't before unless you predicted when, rather than reacted to it. I've defiantly done it to annies before.
You can't really react to the animation itself, but you can to flash because you know whats (probably) going to happen immediately.
If they added more delay to tibbers, she wouldn't be nearly as good at initiating. Her W would be used to initiate instead, and its just a smaller AoE. She would need more damage to compensate or something new to tibbers.
Tibbers being summoned actually doesn't do much damage more than a maxed Q or W at any part of the game. Its him whacking the crap out of you, the AoE burn, and Annie's QW after that which hurts.