"Uber-Keys" (second keyframes) for overall adjustments on existing motion data

You can do (nearly) everything with expressions some how, but it's too slow, to complicated and not intuitive enought for most productive workflows.

After Effects it's a tool for creative person's and not for programmer who fix all weak points of the tool with expressions 😄

I would love to see a + sign beside every animated proparty and if I click it, AE simple adds a second animation layer on top of the old one to add a second animation to the already existing animation.

You can always tie an Expression Controls/Point Control to a layer, set the value to 0, 0, and then use Value +  Effect.Point Control to make small adjustments without fouling up the original tracking data.