short suggestion:
add a .25 second channel to flash.
The potential to interrupt dashes exists in League with Aatrox jumps and Lee Sin kicks against a thresh flay. Every Elo has had it occur with the few champions where it is possible. the perpetrator feels skillful, and the recipient knows exactly what they didn't account for.
A short channel doesn't significantly reduce flash's utility in game.
Flashing out of a situation will give counting CC mind-games similar to nunu's ult and retain the benefit of flashing into a situation will be easier than fiddlestick's ult.
The direction of the flash is undetermined while channeling giving potentially more juke potential despite relative distance traveled being lower.
Final note, I promise, a negated summoner spell may causes some frustration, which may be why barrier > heal. A second option would be an interrupted flash, could go to a 10-30 second cool down.(change the channel to .5 seconds if this is the case).