I want to add my vote for the puzzle ideas: It would be nice to have a puzzle, where the solution is different every time. On top of my head I have the idea of some kind of maze, which looks different every time. (Of course one has to make it so that there always is a way throught
) Some players would have to enter the maze and explore it step by step, while one or two players would have to stay outside and use some controls to transform certain parts of it to open up the way. (similar to the minefield in EC but you need the players inside to explore the area so the players outside the maze can see where they can open a way.)
Another thing I would like to see are situations where the group has to split up into smaller groups that have to complete a certain part of the OP on their own (loot will be found at the point where the meet again), while the successes of each group ease the encounters of the other groups. Again an example: Some players have to stay behind and slow down some enemy that cannot be defeated in the ordinary way (this would probably turn out to be a Tank+Heal duo), while the rest has to go ahead and activate some machine/weapon that will weaken/destroy the otherwise invincible enemy.
This goes into a similar direction as the 1vs1 encounters mentioned already (which I would like to see as well), but also smaller groups that complete smaller tasks of a bigger fight can feel very epic.
Please note that the examples above are far from completely thought through, but they demonstrate the direction I would like to see.