So, here's something I'm considering for a timeline, it's been noted that Edward IV had at one point considered betrothing his daughter Cecily of York to the future James IV of Scotland. For various reasons this marriage didn't go through, but what if it had?
One imagines that it would need to occur during Edward IV's reign, or perhaps during a brief interlude in his son's or brother's reign. Regardless, the marriage happens, James and Cecily are married. What happens next? Does Richard III attempt his coup and declare his brother's children illegititmate? If so, what happens when the Princes in the Tower go missing? Who does who side with? Does Tudor still make a claim for the throne? Does James III invade or is he too busy holding the fort down in Scotland?
If Tudor still claims the throne and somehow defeats Richard, what happens then?