Tessa Thompson, aka Valkyrie chick is unknown.
It sounds like Disney wants to keep the black'ish Nordic Valkyrie as the love interest(lol), rather than Lady Sif, who was also absent from Ragnarok.
I don't think Portman is ever coming back as she wasn't contracted.
Lady Sif was prominent in the comics and gave birth to one of Thor's kids, it's only right they get together as king and queen.
So where's Korg and Tessa?
Well, in Infinity war, when Thor was explaining to the Guardians what they had to do, he said "Thanos already has the power stone because he took it last week when he decimated Xandar, and he just got the Tesseract when he attacked and killed my brother and HALF my people". or something to that effect.
I hope to see them back in some capacity because both were good characters.
They showed what looked like a dead Valkyrie on the Asgardian vessel, but it wasn't Tessa Thompson.
At the beginning of Infinity War it looked like the vessel was broken in half, perhaps escape pods utilized as Thanos was only interested in Loki's position.