Sonic The Hedgehog 4 Can't Bring Back Shadow Without Creating Some Problems

Article: https://screenrant.com/sonic-4-shadow-problem-solo-movie-solution/
they don't HAVE to bring Shadow back in the fourth movie. Same goees for Dr. Robotnik. Dr. Robotnik is the staple villain of the series but he doesn't have to be the antagonist of every film in the series.
Dr. Robotnik is "pressumed dead" now, let another villain take the league and have Dr. Robotnik revealed to be alive by next film, or at least 4's mid-credit or post-credit teaser.
Same can be for Shadow. He's confirmed alive by post-credit teaser, but doesn't mean he has to return straight-away in Sonic 4. But, they can at least give him a TV series to explain his absence in Sonic 4.