1) I think that his invisibility needs to be increased since that way it'll be easier to miss him in a high-intensity setting.
2) I feel like his ultimate should include an assassination possibility where he has a knife (or dagger or something) to stab a player 'from out of the blue', that way you can build off what he has now as an ability, and turn it useful.
3) I think Mirage's mirage should also have a slightly better AI. Maybe the player's movement can help guide how their mirage moves. We already have target locations and crouching, but at the same time, I think we can see that it's not always hard to tell that a mirage isn't real (although maybe within certain - but few- situations). In small spaces, we can't always send a mirage a-running, so maybe we could add AI substance by sending a mirage where he can follow paths around buildings (without interacting with anything of course). Thse paths could be preemptively based on the paths used by players' path history and could be coded into the AI.
4) Just throwing this out there, maybe Mirage can make bluff sounds where he 'opens a door' but actually runs around a building to flank. This one would probably need to be discussed but a trickster can also trick with sound and not only with sight (maybe that could be the case if his mirage passes within a few feet of a door.