One of the major problems with his is that not all ults are equal in power or charge rate. If 50% of a Tracer or Moira ult could be turned into 50% of a Zarya Ult, then we’d have a problem.
A similar idea that could work would be to make Ult charge be retained on switching while still being separate for each hero. So for example: If I had 50% Ult as 76 and swapped to Junkrat I’d have 0% Ult Charge, but if I switched back to 76 I’d be back at 50%
Might help make the more niche heroes thrive a bit more if people can switch to and from them without losing an Ult for it.