Wheatley's return, so many people have already fanficced about it.
Other ones I've seen include GLaDOS pulling him back to torture with an excursion-funnel-like thing and colliding with a satellite and falling back (with the temperature of re-entry not exceeding 4000 "degrees" Kelvin).
They'd need tofind something entirely unprecedented, like adding the gels to P2. Nobody would have expected that after playing P1.
So valve needs to wait for other people to make a great game so they can steal the innovative concept for their game. Then we'll have portal 2 episode 1
Yes, but nobody expected them to add gels after playing P1.
I think nobody expected them to add any other new testing elements, like funnels, lasers, faith plates, etc. after playing P1. Pretty much everything was new in Portal 2.
If I remember correctly from a conversation I had with one of the Valve devs...lasers, tractor beams, and light bridges were items that the original Portal 1 team had experimented with but they didn't have enough time get them to a releasable state. Since Portal 1 was more of a demo, they were keeping it fairly simple as well.
Not sure where they'd take the story next, unless they start with a new protagonist but I'd play it.
Gravity altering, more outdoorsiness, different cubes, colour coded lasers, altering the portal gun etc.
Gravity altering is hard unless you use the PUNT cube. More cubes and outdoorsiness, maybe.