The annihilation of the Boltons and the assassination of Walder Frey are definite reasons to celebrate.
For his part, Bran certainly has some big news for Jon thanks to his trips to the past.
At least Jon and Arya can bond over what they’ve done to avenge the Red Wedding.
Any reunion with Jon Snow would have to be pretty bittersweet for Bran and/or Arya, considering that neither of the younger Starks may have yet heard about what happened to Sansa with Ramsay or what has happened to Rickon.