This is a very popular idea, and one that many players (like myself) who don’t have any of these legendaries would like to see implemented. Basically after a legendary has been introduced it usually comes back for a set period, usually in a single location. For example, Hooh came back for a movie premiere in Japan, Seoul got the legendary birds back for a week, etc: rather than releasing new legendaries constantly can we have just 2 weeks where the birds or the beasts come back? To me it makes a lot of sense to go back to them rather than just skimming through legendaries every month.
Maybe they’ll bring them back for Legendary Year