In the 20 year gap between SoT and GoH happened a lot, not only in character relationships, but in Unova in general. And we will announce the top 5 greatest changes that happened in Unova during those years.

#5 The Festivals

Unova has a bunch of official festivals, like the Nimbasa Festival in June, the Solstice Feast in Icirrus at the day of summer solstice or the Humilau Festival at the end of October. Since 2021, a new Festival is introduced: The Heroes' Festival, taking place in March at the day when Eclipse aka. Addam Kalani was defeated and the heroes saved Alola. Alolan people introduced the feast in 2020 already and it was officially celebrated in Alola and Unova from 2021 on. It became an international festival in 2022 and has hence been celebrated every year.

#4 The Gym Leaders

The Unovan Gym System remained mainly unchanged, except for two things. Clay, the fifth gym leader, retired from his position in 2037 at the age of 62 and got replaced by now 26 years-old Loke Padget from the Kalos Region who travelled through Unova and managed to claim great attention. Even though the Ground Type of the gym was kept up, the change was clearly noticable, since Loke was a lot harder to beat than Clay. In June 2040, Loke however left the Driftveil Gym due to it's ailing state and the unwillingness of Driftveil's mayor to help with restauration, and moved to Icirrus to take over Brycen's old gym, which was left deserted since 2015. Loke changed his typing from ground to fire and became sixth gym leader, bringing in the Flame Badge which will replace the Quake Badge in 2041.
Also, Drayden, who had taken over the dragon-type gym in 2018 when Iris became champion of Unova, is no longer gym leader and mayor of Opelucid. In fact, he died in 2035, since he was already old. For the time being, Chris Edwards had taken over the role of Gym Leader of Opelucid for a while before 2037, when Iris lost her title and took over her grandfather's gym once more.

#3 The League

The Unovan League, on the other hand, has faced drastic changes. Shauntal, the Ghost-Type member of the E4, retired in June 2030 to focus on her work as an author and got replaced by Kaoru Mikami instead. One year later, in September 2031, Marshal, the Fighting-Type member of the E4, followed Shauntal's example and was replaced by Kuraiko Hirose. Ever since that change occured, nobody made it past the E4 until 2036, when Ruuko Steward managed to claim the champion title. With this iron fortress of an E4 and a super strong champion plus the incredibly difficult challenges of White Treehollow and Black Tower, Unova was nominated and elected as the Region with the toughest and hardest league and pseudo-league challenges worldwide.

#2 The Industry

The industry and market of Unova is ruled by Amuro Enterprise since 2039. Many companies have been sold to the giant monster of a corporation. All other big business carriers were weighed down in a period of just a few months, even big ones outside of Unova, like Silph Co., suffered from this massive takeover. Many companies went bankrupt because of it. The Amuro Enterprise's rapid growth is a little controversial, but a huge success as they are about to expand to Kanto and Johto, ready to take over the whole world's market entirely.

#1 Illegal Groups

Even if Unova was flourishing with positive growth, it was also facing a dark side of just that. One of them is the uprising of the Ugoya Cult in May 2039 which was incredibly difficult to control and dispose of due to the leader's cleverness and his many spies. The cult was feared, but also kind of distracted from the real threat lingering underground. The real threat are the 5 mafia groups which have formed in the canalisation of Castelia since 2037, flooding the surface with false money, drugs and dangerous Pokémon.

And through we are! I hope this was rather informative for you! Leave your opinions in the comments!