I’m personally not a fan of this idea. The name of the game is RNG. They’re also legendary Pokémon being handed to us. People also forget that Niantic is here to make $$$.
If we were able to narrow it down like that then Niantic wouldn’t have people coming back every day to spin the stop, to get the research which builds up to the 7 day stamp. That’s 14 in game actions per week over (potentially) 3 weeks which would otherwise not be done by most after obtaining the legendary they want (as they’d get it then not have to come back until they could pick a new legendary from research).
The less people play, the less revenue Niantic makes, less revenue = less features they can provide & ultimately the game becomes uneconomically feasible.
Coming from a position where I’ve put in a lot of effort to catch those legendaries that I want plus any available shiny variants I think it would be unfair for us to simply “pick” what we want from a legendary selection.
I’ve also had my fair share of rubbish Regis & no shinies from the research since they came out either but when one gets released that I want, another 3 day event like the recent Groudon event or legendary raid day happens (shiny Rayquaza, I’m looking at you) I’m sure my raid count will be going up from the 50+ RayRays I’ve done to near the 100 that I’ve done to get my 2 shiny Groudon.
Again personal opinion 🥴