Keep in mind that it's not the bubble that's powerful on it, so much as the fact that it has a good chunk of HEALTH, MANA *AND* MAGIC RESIST.
These three things are exceptionally strong on virtually every champ, especially the magic resist + health combo.
It's good enough to the point that it's sometimes worth getting two banshee's veils over other magic resist items in some situations, despite the fact that the passive is unique.
The sheild's nice as well of course but can be easily removed. On the other hand, not every group happens to have highly spammable aoe either. No avinia, ashe, etc, and it kind of grows significantly in power.
Unfortunately, the sheild is one of those broken mechanics that can't be properly balanced no matter how many tweaks yeu make to it. It'll either always be too powerful or too weak with no real middle ground.
The only thing I can think of to make it even remotely balanced on the shield part, is make it only affect single target spells, but increase the cooldown to 45 seconds up from 30. And even that's not going to fully fix it since it literally is unfixable to a degree.