I would love a Portal 3.
What would you really add or change that would warrant a third game though? The second game can already be expanded into multiple full games thanks to the mod community.
The first game laid out the concept, the second game refined it and expanded on it. While technically nothing stopping then from making more of exactly the same game such would be disappointing against most people their expectation of improvement and would be easily criticized from all kinds of corners. Weather it is fair or makes any logical sense valve today is who they are part because of their imago, maintaining there respectable and high quality standard is economic survival.
I don’t know what I’d add, I’m not nearly creative or talented enough to design games. I just love the world and want more official stories in it.
I trust Valve to only do it if they have something that would be as big a jump as there was between 1 and 2.
There’s something similar, in Portal Reloaded you can travel between timelines. It’s a mod, though: https://store.steampowered.com/app/1255980/Portal_Reloaded/