I will always wish for Portal three I hope for it to become a real game but we will have to wait I'm guessing valve might be doing Portal 3 Half Life 3 and possible Team Fortress 3 in VR just a guess.
probably not, the story seems to be quite finished, considering the coop missions take place after - that kind of is the 2.5 part of portal and probably the final ending, valve is making tons of cash anyways by selling other peoples games, so they dont need to make some themselves