Just imagine an Earth day event where trainers can access Mr. Mime, Tauros, Farfetch’d, and Kangaskhan in their region. They won’t have enough time to collect candy for these Pokemon and having this event defeats the purpose of having regional Pokemon, but with the lack of Trading, this could solve that issue of the incomplete Pokedex.
A Massive amount of trainers around the world will probably not be able to travel all over the world from fear of planes or boats to complete their Pokedex, and a good amount of trainers will not travel due to living situations which makes the feature of regional Pokemon unfair for them. Besides having this event will open the opportunity to add more regional Pokemon to the game from future generations. Let us know what you think about this idea in the comment section below! (Idea by pmmckee on Reddit!)