I'd like it...but I doubt they would...
If they were to it would be like 12 years from now. HOWEVER, you know how quite a few people didn't know about the edited ending of the first one until recently? Well I can see that happening again only with like a co-op patch to add more levels. In fact I could see a DLC where Atlas/Peabody or both (or another robot) have to go capture Chell OR a relative, since she may have her own life now.
My second suspicion would be that when Chell is left in the outside world...it's actually a cruel surprise and the cube was necessary for a puzzle. AGAIN quite unlikely, but when I got to that outside, I looked at the skies, looked at the wheat and thought, "Hang on, this looks slightly dome-ish...OH GOD I've not escaped have I?" shortly followed by "OH LOOK, CUBE!!!"
Loved the ending though. Half Life 3 first, then probably a new game, Left 4 Dead 4 will happen I can bet (I know 3 isn't out yet but I doubt they're gonna pass up on the double 4). Portal will be forgotten for a bit, but if they did do a third, I can see it doing a Toy Story 3, but it needs to be forgotten first and quite frankly, I just downloaded the song "Want You Gone" onto my ipod so it will not be dying anytime soon that's for sure...
<listens to GLaDOS' beautiful singing voice>
EDIT: OMFG just realised another possibility. Anyone complete the co-op yet? Anyone realise they had loads of comatose humans that they're bound to end up testing? Well what if they sent in Gordan Freeman to free them all. I KNOW it's unlikely, but this is valve we're talking about, they've been hinting at it all this time. Chell could have left and is no longer in the picture but GORDAN is still firmly in the picture. Some sort of cross-over for Portal 3 and Half Life 3...I can actually see that happening now...