On the PC version (top, right side- under local time/poke time) there is a tab where you can see which Pokémon are in the area, and if you click on them it gives you more in detail about them
in Pokedex via stats, where to find them, etc.
Some people forget this feature or simply don't use.
I'm not sure what it's like for mobile users as I don't use mobile. (Could be hard for mobile users if they don't have the same layout)
However, what I would like to suggest is- when a shiny/event 'mon is close in the area the respective 'mon could turn a different color perhaps in tab?
I think this is very feasible seeing as it's already a tool that anyone could use.
It doesn't have to tell us where it is to keep the mystery. But just to let us know we're getting closer to it.
Collectively, I think it would be helpful all around, esp. for those who don't have great luck & don't catch or have never caught event/shiny as easily as others.