By using these 5 pokemon they have teased for the 25th anniversary set and looking at 28 pokemon in the set as well as more expensive packs, I have a theory. If you look at all of the mascots from each generation of the 25 years, it adds up to 28 (Not including the remake repeats). Starting from most recent (Calyrex, Urshifu, Zamazenta, Zacian, Eevee from Let's Go Eevee, Dawn Wings Necrozma, Dusk Mane Necrozma, Lunala, Solgaleo, Yveltal, Xerneas, White Kyurem, Black Kyurem, Zekrom, Reshiram, Giratina, Palkia, Dialga, Rayquaza, Kyogre, Groudon, Suicune, Lugia, Ho-Oh, Blastoise, Venusaur, Charizard, and of course Pikachu as the mascot for all of Pokemon).
Obviously it could be something completely different but what better 25th anniversary set than the heavy hitters from each generation.
https://bulbapedia.bulbagarden.net/wiki/Game_mascot#Generation_VIII