A space drama would be over budget for any night-drama. They'd have to CG a background for the whole thing, or have a really boring white room.
An Edo theme is kinda interesting. Kinda magnifying out Nogi46's Tsuki no Ookisa MV into a story. To film more easily, maybe use edo-type charas in a modern setting (e.g. like the live-action Fuma no Kojiro).
The asylum breakout might be a little like Joker Game Escape.
http://asianwiki.com/Joker_Game_EscapeJust take away the 'torture room' acid traps/clue-searching, and add a bunch of fistfights. Do-able, but I think this resembles the theme of MG3 too much. And honestly, the detention center setting wasn't the best change for the show. The battle arena was great, but it came up way too late and wasn't used nearly enough. They should pretty much stay away from this.
As in my previous post. I hope they continue from the ending of MG3. With the girls trying to survive in the city. And meeting/challenging the MG1/MG2 characters, linking up the 2 universes.
Its got the "Majisuka Gakuen" title, so they should wear school uniforms again (i can easily come up with 12 unnecessary storyline-reasons off the top of my head). And they'll be learning life lessons on the streets. So it totally fits the "gakuen" theme... more than a detention center at least.
Its easy for us to come up with crazy ideas with our imagination though, totally different story when its the guys that are actually thinking it up and filming it.
The strongest point of MG is the build up of each member's character. And I think they do a great job with this. So the setting really doesn't really matter that much.