I think you are going about this the wrong way.
Ulti at 5 and max lvl by creeping at 4 does not solve the core issue - that some lvl 3 skills are better than some Ulti’s (MK Bolt an example that was given earlier in this thread)
It would change a bit since some heroes would be forced to choose at lvl 5 between ult and lvl3 skill but some heroes like the Death Knight would just be f***ed over by this.