Be able to define and use alpha values for colours
It would be really nice to be able to set the alpha value in colour pickers! When using expressions, AE already has the value there as the array for colours is [R, G, B, A], but changing the alpha value doesn't actually do anything to the produced colour.
I know that opacity is already a property, but it could be useful to be able to control opacity through the colour value in addition. For instance, if making a null solid, the colour could be set to [1,1,1,0] from the colour menu and not require the opacity property to be set to 0 each time it's dragged into a comp. The colour value on a Fill effect could be set to [1,0,0,.5] and produce a 50% red overlay on the layer. Tint/Tritone could turn shades to 0% opacity without needing a matte or other effect. Or even just being able to control a shape layer's alpha through the same property, without having to split things off to a second expression on the opacity property.