How to balance Hextech Starburst
Remove the damage from the active, increase the dash range slightly. Done.
Champions with mobility won't buy it because "GLP-07A" (is that its real name?) is better for them in every way. Champions that need the mobility will still buy it, for the same reason people buy the Blink Dagger in DOTA despite it not even giving any stats.
This item's passive is already super good on the champions it's meant for without doing damage. By adding damage you're just opening it up to be abused by champs like LeBlanc and Ahri.