Admit it. Playing Sonic the Hedgehog games competitively against others is fun. Both Adventure games, Heroes, and the 3DS version of Generations all feature local multiplayer, and in the case of the 3DS, online play.
I have been keen to booting up Adventure 2 and Heroes when mates are round, and all enjoy it. But modern Sonic games seem to have just become single player affairs.
Imagine how cool it would be, playing with multiple characters on a multiplayer mode. Now this exact scene might not be able to be played - after all, City Escape of Adventure 2 cut the last part of its level just after running down the building for the ease of access. After all, multiple GUN trucks on that last part wouldn't be advised. Just like multiple choppers jumping at the scene in that picture wouldn't be advised. But why couldn't that part be bypassed.
After all, a new level file needs to be placed if even the slightest change to a level is made, meaning both co-exist peacefully and depending on the number of players one file is swapped for the other. And there are multiple routes to take, and even if not, they can be made for a possible second player.
Take Green Forest and its vines. Because only one person can use the single vine, springs were placed to allow the second player to pass without waiting. For Green Hill on Generations then, the underpass could have been bypassed for multiplayer sake. I wouldn't know exactly, but surely the part above the waterfall would have made an excellent choice.
As for later levels, Chemical Plant is fully playable, Sky Sanctuary cut just before the falling path. Speed Highway needs no editing, just like Seaside Hill. City Escape, again cut just before the GUN truck part. Crisis City would be a bit more difficult for the player behind, but can be rectified with jumps instead of the usual placement. Rooftop Run is fully playable except for the last part with the blimp and lasers. Planet Wisp can be cut before the dropping walls at the end, or better yet, don't make them fall.
As for other games, Unleashed as far as I can tell would have been good for multiplayer. Colours I think had some form of multiplayer, as did Black Knight. Sonic 06 would have been terrible for online, even though that was the last game to feature other playable characters in the main story (Black Knight had other characters playable, but were optional).
The appeal for multiplayer is there, but due to the lack of other playable characters (which to be fair isn't an excuse) SEGA deem it not necessary. Now you might have noticed I was only talking about the modern levels. And that's because handheld Sonic games have always been supported with multiplayer. Which to be fair, doesn't offer the competitiveness of a 3D Sonic multiplayer.
I seriously hope that the new Sonic game features multiplayer and goes even further with online multiplayer (not counting leaderboards, like with the HD remake of Adventure 2 - online could have further increased the reason to buy), but even if the new one doesn't, it will still be an essential purchase. Multiplayer will make it more desirable, though.
SEGA usually announces Sonic the Hedgehog games in April/May to be released at the end of the year. When official news is heard, my opinions will be uploaded straight away.