Addons cannot interact with the 3D world in any way, so things like "glows" or "targeting circles" on mobs in the 3D world aren't possible. Honestly, nameplates are probably the closest you can get to that, with all their various drawbacks (not appearing at all on large/close or distance mobs, jumping around in crowds, etc.). It may be possible to make the target nameplate stand out even more than it usually does, but that's probably the best you can do with addons.
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