This is sort of a superset of HoTBar, which I think should be part of the default UI (and should also be Aced), but that's another story.
Here's what I want:
When I (as a priest) mouse over a heal, I want my target's hit point bar to show me (similar to how HoTBar does it...wich a different colored bar extension to the HP) how much the person is likely to be healed by the spell. I'd like it if it somehow worked with BeneCast, GroupButtons, etc, as well. In fact, every time I mouse over a heal, what it should do is show how much that would heal everyone in my party, myself, my target, and I suppose my raid as well. So I can do a quick mouse-over each of my heal buttons and see "oh, that'd over heal my mage, and it'd get my warrior up to 80%..."
As for how it deals with the min vs. max vs. crit-range of the heal, I leave that to the implementor of the mod, I just want to have a good visual estimate... ('cause, let's be honest, "heals 960 to 1040" really has no meaning...unless I actually check "oh, my warrior has 3424 HP, so it's sort of like a third of his HP..." but even that's hard to visualize... It'd just be so sweet if the bar just showed me. :)