Two champions have fixed attack speed...
And both are interesting, entertaining, unique, fun and meta.
Vayne could have her damage somewhat fixed, she could be entirely focused on beast hunting.
Right now though tanks aren't a big thing so it'd just kick her to the curb, but maybe once you fix the problem that every champion has to be an assassin, her role could be defined as a hunter, tracer and mainly tank buster.
Different ammunition could have different effects, like one could leave "open wounds" and once they disappear into the fog of war you'd see a blood trail for a few seconds. No huge ADC crits but no standart auto damage either.
I don't say this out of any pro or con bias. I just think we could use more Jhin or Graves like champions, in this case ADCs. And Vayne beast hunting fantasy is fun to explore