How about an addon that is similar to Healbot except that it would show all mobs that members of the raid are targeting and display them with health and dot timers. It would allow someone to click to apply dots, use pestilence, etc to mobs that may be hard to target in the chaos that is combat.
I guess what I'm really thinking about is a frame that would show all the mobs that are aggro'd and/or targeting myself or members of my group/raid. From that frame I would then click to attack, cast, or apply dots. Recently acquired the Naga mouse and found that it's extremely powerful but mouseover targeting and casting is nearly useless in most fights as a dps. Would be very useful to be able to use one hand for wasd movement while casting, dot'ng, targets that are sorted according to distance, remaining health, dps output, mob type (healer, caster, melee), etc.
It's not possible for interactable (eg. clickable) frames to show arbitrary mobs. Frames can only show mobs with valid Unit IDs, such as raid4target or even raidpet14targettarget. Frames also can't be created in combat, which is why you'd normally use a predefined set such as "raidNtarget" frames. Even if you set up raidtarget frames, you'd see a lot of duplicates, since at any given moment, the majority of your raid is probably targetting the same mob.
Basically, what you want to do is probably something Blizzard would rather you not be able to do -- just spam click on some boxes to maximize your performance. They don't even like healers doing it. :p