It's sad to leave Roshar for the next decade or so, but I think the endings of Stormlight 5 and The Lost Metal indicate the new Cosmere conflict with Retribution is going to play a part in Mistborn Era 3.
I think we can assume the group of people MeLaan sails to in Shadesmar is the same group of people Shallan is with at the end of Wind and Truth. This makes it very possible Shallan and MeLaan meet. Shallan rejected the Ghostbloods, but could we see her become an agent for Harmony? It seems unlikely due to what would happen if she left the time bubble, but could we see Shallan on Scadrial?
The time bubble is a neat trick that will allow Brandon to do some cool stuff. It means it's possible the mortal characters from Mistborn Era 3 have interactions with the Stormlight mortal characters we already know. Could some of these Mistborn characters take part in future Stormlight books? Another cool thing the time bubble will allow is scifantasy elements in the 2nd arc of Stormlight. It would be pretty rad to have a special ops team from Scadrial, fully equipped with guns, night vision, and advanced allomanticly-powered weapons and gadgets, go on a mission to Roshar where they face enemies with swords and bows.