I'm not sure how sega can grow in the worldwide market, their current ip offerings are either niche pc games or niche Japanese games (or sonic). They need a true overhaul of their game development arms/studios to have any chance of being a major publisher again.
Start by understanding what it takes to make a good Sonic game, then work outwards from there.
Increased UX maturity would be a big step in the right direction. Put your ideas in front of players and work together to understand what players actually enjoy, what they want from the series, what's working, what isn't. Put the older games in front of players, ask what they like about them, put games like Forces in front of players and make an effort to understand - in fine grained detail - why the games design just doesn't work. Look at what their competitors are doing, what makes people like those games, instead of Sonic games? Reshuffle management at their studios. Promote open communication between their developers and development teams.
Sonic Mania is fantastic but they lucked out on having Christian Whitehead, someone that does understand what it takes to make a good sonic game, willing to do the work. When the guy that best understands the Sonic series is someone that reverse engineered the fun out of the game (Christian started by making Sonic fan games), then that's a sign that your internal studios have some relatively pervasive problems.