An addon could be made to:
- replace or add to the nameplate graphics of the current target a reticle graphic.
This does not change the targeting mechanics at all, you'd still need to click or tab to a target, it would just make the target stand out more.
- poll mouse-over and replace the cursor graphic with a targeting reticle when the mouse-over is not also your current target.
This also does not change the targeting mechanics at all, you'd still need to actually click or tab to the new target.
What the original poster wants (reading as much between the lines) is a way to
tie targeting to the camera so that you could for example
press a key-bind to target whatever is under the reticle while mouse-turning (ie no mouse cursor is visible)
While this: "being able to target using the mouse and a key-bind without going out of mouse-turning mode even momentarily" would indeed be a groundbreaking change for PvP (and not only)
it is also as Seerah already pointed out
absolutely impossible to make.
It is not supported by the game engine is another way to put it.
It's not a matter of the idea not having come up before or the willingness / coding ability of addon authors...
it is literally not possible within WoW.
Edit: It might however be a good idea to post in official suggestions forum as something to be added to the game for the next expansion
