One versus All modes in GE:S don't work because of the temporary invulnerability you get after getting shot. It makes it unfair for the group of players since only some of their shots are counting. It's a core game mechanic and it just wouldn't be GE:S without it.
As for a lot of gameplays being similar, I think OHMSS is similar to GunGame by design. Casino Royale is somewhat similar, true, but Uplink is definitely new and fresh. 4.2 code offers a lot more flexibility (be prepared for a sexier Uplink and a few other gameplays from me) and should open the door for more unique gameplays.
A lot of people suggest gameplays, but the trick is working with the limits of the GE:S code and mechanics. There's a lot of room there for gameplays, but understanding what you can use is important. If any of you want to start up your own gameplay, I recommend this book for learning Python.
http://www.greenteapress.com/thinkpython/
