What impact would it have had on Scottish history if Margaret of Norway hadn't died in 1290? The Scots would have rallied to her but Edward l would still be backing(IMO) John Balliol to be King of Scotland. I think Edward l would still have invaded Scotland just like he did in this reality(John would wind up not being good enough for Edward Longshanks)and named himself King of Scotland.
Margaret Maid of Norway was supposed to marry Edward of Caernarfon - King Edward's oldest surviving son - in a marriage arranged between the English King and Margaret's father Eric, King of Norway.
This was a deal negotiated without the conclusion of the Guardians of Scotland and they, as a collective entity, signed a declaration accepting Margaret as their Queen but refusing to accept any marriage proposal until she was actually crowned.
The Balliol and Bruce's claims to the throne were still a major matter of Scottish politics at the time and not all of them would have accepted her as their monarch, especially if it meant accepting her meant accepting a King none of them approved of.
As a result it is not beyond the realm of all possibility that Margaret could have arrived in Scotland only to vanish before her coronation and never be heard from again, assassinated on behalf of one of the other claimants. This would, of course, plunge Scotland into civil strife if not civil war, with each faction blaming the other for her dissapearance.