DOOD, yes, you can create said items with thok item and extend them with a timer soc, but then why don't we see more wads like that? Because there isn't a point to them and are basically broken in match. Super Sonic drains rings because you aren't suppose to go super for the rest of the stage. It's just an extra boost to help out in certain parts. The idea you suggested leaves no room for normal play and just make the game boring.
I'm pretty sure it's because most power-hungry noobs don't know how to, hence the popularity of the various SMS wads in 1.09. I'm certain that most coops that aim for Super Sonic use setrings 9999 anyway.
As for the emerald removal, you loose ONE game element, and suddenly the ENTIRE GAME is ruined? I mean, do you NEED all those emmys? They're too easy to get in Coop, why SHOULD you get Super Sonic? I mean, if emeralds were just INCAPABLE of Superness in coop aside from Pandoraing it, I'd be fine with it. I mean, why bother with Supers in coop anyway? You have several people around, why not...well...COOPERATE? Isn't that what it's for? COOPERATION? Anyone remember that? Aside from begging the serv for setrings, there is NO coop in coop. So why bother making even LESS coop in coop by adding Super Sonic?
Now, I, and probably a few others, do NOT want Super Sonic to lose his uniqueness. But at the same time, I do NOT want people just using Super Sonic in coop all the time. The solution is simple; eliminate Super ANYTHING in coop. Emeralds can stay for collecting (and that ONE thing in CEZ2, the ability to skip the dungeon segment and open the wooden gate), but that's about it. Like I said, you already have game breaking abilities via char wads and switching at will (Which I also don't like, but what can we do about that, make chars only able to switch at checkpoints?), so why get Super? Just take him out coop, except for that Pandora's box option, which is mainly just there for entertainment anyway, like all the other options in it.