I'm just getting into the game, so I haven't played all that much. I haven't even played through the official levels, so I've mainly been playing with the add on ones. So as a complete noob, I'll give my thoughts.
Personally, I like it when the levels are more linear. I've found myself getting disoriented by many of the stages in trying to figure out where to go. Sometimes I will miss something obvious, like springs that I'm supposed to hit to get to the next area. I would much prefer if levels all pointed to a specific direction for you to go, so at least I had a heading. Sometimes levels will loop back on themselves and then I get really lost.
I also feel many of the levels, especially the official levels I've played, are all too cramped for how fast you go. Many times I find myself running into walls or off of platforms because I couldn't see around a corner and react fast enough. I get frustrated having to play stop and go all the time. That kind of gameplay would be fine for any other platforming game, but I want to play Sonic where you go fast.
Watching videos of people playing, it seems the most optimal play has you jumping and dashing from point to point until you get to the end. You can often avoid entire parts of the levels without even touching the ground. At least for me, that's not how I want to play. My ideal map would be very linear, but with branching paths that converge back onto the main line. Some of those paths would be ones that cater to different characters' abilities, like flying over pits or destroying walls. It would have large enough spaces that allow you to see what is up ahead to plan on your movements. Or if the spaces are narrow, have more indicators to show me what is up ahead (like arrows or something).
I guess this kind of sounds more like a racing game in a lot of ways. You have one main path to the end of the course, with a few shortcuts and alternate paths thrown in, but they all converge back on the main path. I know there are the kart racing spinoffs, but I would much rather have a map designed like this with actual Sonic gameplay in mind instead of kart racing physics.
The maps I've found that are close to this are the Speed Highway and Nu Angel Island ones. Anyway, hope this helps!