She's not really an assassin man. She's a battle mage and should flourish in prolonged engagements. Diana has the second best clear of any champion that I've personally played. And if built to utilize her passive does indeed have the power to hang tight with bruisers and tanks in close quarters combat. There are plenty of hybrid ap/defense items that can be built. She doesn't seem like she was ever intended to be a pure mage to me. If people started building her for sustain in combat she would be way better off.
I don't think we should try to stack ap on every ap mid simply because they can mid. She's an excellent jungle/top lane pick as well. And far closer to being a mage bruiser than she is a "Stack over 9000 ap" champion. Changing her kit would be a mistake I think. Changing her recommended. Would not.
And your change is extremely op. I would rather see it be a skillshot jump if we're talking about that line of reason. Not a targeted. But then you risk losing all of her damage potential.