The aftermath of the reign of Robert Bruce could have gone better if Edward III had died early, and, in fact, there could have been a union of the crowns much earlier than 1603 if Edward III and his brother had died without issue and sister Joan, wife of David II of Scotland, had inherited the throne and had children of her own. There's always a possibility of a different union: with a slightly earlier POD, the child Margaret could have been the heir of Scotland and Norway, and something like that could have happened later.
If there's no union, it's hard to see Scotland doing all that much better territorially than it did anyway and the other posts in this thread have run through the possibilities.