Currently most events such as EVENT_EFFECT_CHANGED and EVENT_COMBAT_EVENT use the unitId to identify where various effects are originating, however when you're actually standing there looking at a specific unit, there is no way to determine which UnitId you're actually interacting with. This leads to addon devs needing to implement a number of super hacky and not particularly performant workarounds (lookup tables) to synthesize information between the events and the player's target. (We have to do this in Fancy Action Bar+ to determine who the heck you still have your debuffs on when you try to update the timers based on your targets, it's.... gross).
I think adding the UnitId as a return from EVENT_RETICLE_TARGET_CHANGED, which doesn't currently have any returns (at least not in the documentation), would be an amazing way to allow us to get this information which there currently isn't an easy way to get, without us needing to resort to these awful workarounds.
Alternatively (or additionally), a simple API function like GetUnitNameHighlightedByReticle() but for UnitId would be a welcome addition.
This would also address the scenario described in:
https://www.esoui.com/forums/showthread.php?t=11031