I've played a lot of Mirage and I think you'll never get people to agree on what they want her to be. Here is my personal take on what I think Mirage needs to feel cohesive. I envision her as a skirmisher playing with the light to both confuse and hide from her enemies. Randomness has been a part of her kit with the ball going wherever it wants and Eclipse not behaving, but I think it would be more beneficial gameplay wise for that to no longer be the case.
Mirage should be set to Light or Dark mode based on Ability cast of Hall of Mirrors or Eclipse and have the corresponding Icon above the ability bar for quick reference. Light/Dark mode abilities are all or nothing when active.
Hall of Mirrors
Now re-castable
Puts Mirage in light mode
Contains the Light buff from Eclipse
If eclipse is active, Blinds enemies in within 5m for 2 seconds
Turns off Eclipse
Sleight of Hand
I don't really care about this ability, IMO something better could go here, but if it stays w/e. People that play SO/ESO will complain so I get if you can't change it.
Alternative Mirage manipulating Light in her environment and it would remove the mutual exclusivity between HoM and Eclipse I'm proposing by creating zones of Light that blind enemies that walk in or zones of Dark that Enemies can't really see through. Zones could be a press/hold like Vauban 4 starting as dark and the turning on the light or vice versa
Eclipse
If Hall of Mirrors is active, become invisible for 2 seconds
Turns off Hall of Mirrors
Puts Mirage in dark mode
Maintains DR for remaining duration
Prism
Light mode Prism acts the exact same, except will follow the cursor while aiming.
Dark mode Prism creates and area of darkness that absorbs incoming fire making you and your allies harder to hit.
This is my rough outline, but I'm just hoping you do right by Mirage with whatever changes you make for us players.