Put aside your comments about this idea is bad and will never happen. I just want to discuss this for the fun of it.
Assume the building ubercharge mechanic works the same as it does in tf2. Would she be more interesting to play? Would that be bad for balance? Would this make her too powerful? How would you balance this? What do you think Mercy’s ubercharge would do?
For those who haven’t played tf2:
“When healing, the Medic will gradually fill a unique ÜberCharge bar, which can fill faster if healing injured or not-overhealed teammates. When the ÜberCharge bar is fully charged, the Medic’s healing gun will begin to crackle, accompanied by small electric Team-colored particles at the tip of the Medi Gun, indicating that he can now deploy a unique charge to benefit his healing target, and himself, for eight seconds.”(tf2 Wiki). Think of it as a smaller scale ultimate that buffs you and your healing target in some way that you lose if you die. Some examples of what an ubercharge does is grant invulnerability, heal enormously fast, triple the damage output of your target, or grant dmg reduction.
(Disclaimer: this is not a serious balance suggestion. Just a fun topic I’d like to discuss.)
I would especially like to see how you Mercy mains would balance the ubercharge if she was given one.