Honestly i'd rather ask that her w passive proc makes it so her passive remains up for the duration (or some specific amount of time). So she has some reliable mobility within combat. I'd wanna avoid anything that gives per more poke power as her q can be devastating in a laning situation.
But i do agree on the ult thing it is useless when a single aa would be more effective (though i'd ask to see what adding on hit effects to each segment of her ult will do- though make it more effective per rank e.g.
At rank one: applies on hit effects every 3 shots.
At rank two: applies on hit effects every second shot.
At rank three: applies on hit effects every shot.
This would be sufficient to make MF a more diverse champ (the item builds would be rather amusing) while having her ult become more useful (not to mention give mf a pretty cool tool to use).
But what i suggested may just break her... Im not always good with balancing fun with Riot's idea of balance.