Let's see if I can explain this.

When an addon creates a button that casts spells, it has to specify what unit that spell is going to be cast on.  It can't change that during combat; only outside of combat.

The units that an addon is allowed to specify are a fixed list that Blizzard has created:  target, focus, player, party1 through party4, raid1 through raid40, pet, arena1 through arena5, boss1 through whatever the limit on the number of bosses is, and unit names that can be created by adding "target" one or more times to those (e.g., "targettarget", "pettarget", etc.)

So.  For general usage, the only enemy targets there are target and focus.  It's theoretically possible to do some tricks with pettarget, party1target, etc., but your party or raid members could change their targets, so that's not reliable unless you're multiboxing.  Pettarget can be reliably worked with, but suffers the limitation that you're stuck keeping your pet on a single target.

Healbot can give you a button that you can always use to heal yourself because it can assign that button the "player" unit.  Similarly, it can use "party1" etc. to give you buttons to heal your party members.

But for enemies, you don't have unit names like that which refer to a single unit and don't change.  Instead, you have target and focus.  Target changes every time you target a new enemy.  Focus changes only when you make it change, but there's only the one focus unit.

So, for your addon idea:  You target something and this new window pops up.  The new window has to refer to the enemy as "target" or "focus", because that's the way the system works.  If it refers to them as "target", then it will switch to your new target whenever you change targets.  If it refers to them as "focus", then you have to have them set as your focus, and it'll switch to your new focus is you change your focus.

You can't make a new window that refers to an enemy and will *keep* referring to that enemy, except with focus, and you only get one focus.  Thus, you could only have one such window.

In theory, you could have code that looks for another way to refer to the target -- e.g., checks the boss units to see if any of them are pointing to that, checks party1target, party2target, etc... but boss units only exist in certain boss fights, and using "party1target" would mean that the window would switch to party1's new target whenever party1 changes targets.  As I said above, if you're multiboxing, you could theoretically have each of your five guys target a different enemy and refer to them like that, but that would be a highly specialized addon.

Believe me, the idea has been proposed many times before.  I primarily play 'locks, so if such an addon could be written, I'd have already written it.  But it can't be within the limitations that Blizzard has set, which is why no one has already written one to do it.

That's possible and legal.  Xtreme Unit Buttons and Groupbuttons were addons that did basically that -- giving you buttons that attached to the unit frame which you could click to cast spells.  Not sure if either's being maintained any more, though.