As much as I liked the Adventure games, Sonic Team and the other devs who work on Sonic need to try not to focus on story so much and devote more resources to gameplay and design when making games. Most of the bad games in the series suffered from too much focus being put on the story in one way or another, including the most infamous among them (Sonic 06 and Boom: Rise of Lyric). Get more technical people in there to work out the bugs, and more designers to craft longer, more engaging stages. Gameplay-wise, they also don't have to reinvent the wheel every three games. The "boost" style of the Unleashed day stages, Colors and Generations worked well even if naysayers were all like "boost to win", and relatively minor gimmicks such as the wisps were all you needed to mix it up and keep things entertaining. Going back to this gameplay formula was not among the problems with Forces; heck, one of the problems was that Forces didn't do it enough, the stages were too short and when you weren't playing that style of stage you had to do the (apparently) super-wonky 2D sections (I honestly didn't have a hard time with the 2D stages though if many did that's an issue). Mario works well following these rules: the story stays simple (even if repetitive, but nobody really plays Mario for the story); gameplay has a solid and established foundation for 2D and 3D and innovation/change in gameplay builds on that foundation instead of scrapping it and starting over; and level design keeps players engaged in gameplay with right-sized stages to be substantial but not overly short or long. Most of the 2D games do usually get this right, but why they can't translate that consistently to the 3D games (and there have been good 3D games like Adventure 2, Colors, Generations, and at least Adventure 1 and Heroes were decent enough) I have no idea.