Many consider common knowledge that - Robb designated Jon as his heir in a document usually called Robb's will - This document was given to Robart Glover or Maege Mormont who were going to visit Howland Reed Then whole theories based on that developp ideas like a Great Northern Conspiracy to crown Jon, etc... to explain why the will was never spoken about again (so far). It's well and good, but sadly there is no textual evidence of the two "facts" above. So let's consider another scenario.