Now, I'm not arguing against allowing spectators in modes that don't allow them currently, but I really don't think that allowing spectators will solve the problems that you guys are having. All spectator would do is allow you to have someone easily go AFK without disrupting or leaving the server.
The core issue here is that Cooperative mode isn't really designed with any gameplay style in mind, yet everyone plays it with random people they don't know on the master server, all of which have differing opinions about how the mode should be played. Some people want to rush to the end, some people want to spend time exploring and breaking every block in the DSZ2 block room, and some people just want to chat (I don't really know why they're using SRB2 to do this, but they are). These differing play styles aren't going to magically come together to cooperate because the game doesn't force them, and there isn't an easy fix for that.
Imagine New Super Mario Brothers Wii/U's multiplayer. Notice how in those games, while you're moving, you're FORCED to move as a group because of the camera limitations. You can't just go off on your own and explore some corner of the map because the other three players will scroll you off the edge of the screen and kill you. Also notice how most of the fun of this mode is watching as people's differing play styles of Mario games gets themselves and their friends killed in amusing ways. The camera structure essentially forces you to play as a group, and then by being as a group you constantly interact with each other, making the mode interesting and different from single player.
SRB2's coop, having a separate screen for each player, doesn't do this. There's absolutely nothing encouraging you to stand by your friends, and most players just run off in separate directions. Because the game is 3D, it wouldn't be practical to limit the distance you can separate from each other in that way, and putting in the other similar suggestions like a giant multiplayer combi-ring would be far more effort than we're willing to spend on the gametype. This means that the gameplay is whatever you as a group make of it, and if one person wants to thok through shit and the other wants to explore, there's going to be an obvious conflict of interest as the player who dashed through the stage waits for the other players.
I've said multiple times in the past that Cooperative mode in SRB2 is a bad mode for online play, and this is the reason why. Offline on a LAN, or with a group of real life friends on voice chat, cooperative can be a lot of fun. You coordinate with your group and help each other through the stage, like the name of the mode implies, not because the game tells you to but because you're friends and you WANT to. The best fun I've had with Coop SRB2 was using Tails to help a lesser skilled Sonic player through the main game. There's no way to put that friendship and cooperation into a random group of strangers on the master server, and no feature you request is going to fix this issue.