So I think a lot of what we see in Star Wars Legacy could in some way still happen particularly the Rule of One. I still think Darth Krayt could one day appear and establish his sith order. Now it'd take some minor changes but I do think it'd still work for Krayt to appear. Honestly, I feel like if Baylan can exist so can Krayt. Cade Skywalker in my opinion can work as a descendant of Rey or they can just say Luke had a girlfriend or whatever off screen it doesn't have to be overly complicated.

For me I think a good way of reintroducing Krayt would be for him to be the boy who asks "What are we going to do?" just retcon that he survived like Reva and was carried out by an older jedi. To be honest I think making Vader OP has done him a disservice. That's why I like the idea of Krayt returning to canon as one of his Vader's victims and just absolutely giving Vader the fight of his life.

As for where he's been I can see it being as simple as some jedi escaping on a ship with some younglings crashing on to Korriban and being stranded for a long period of time.