Let's say Elizabeth names Arbella Stuart as her heiress. That butterflies away the Union of Scotland and England for the time being. She marries Edward Seymour like she almost did IOTL and the two have issue.
But then in Scotland, James (son of Mary, Queen of Scots) dies before having issue. Therefore a succession crisis happens.
I know Arbella had a claim, but the goal of this isn't to have a United Kingdom of Great Britain. So instead, is there either someone with a better claim than her (perhaps a Hamilton or de Guise?) or a good enough claim to fight against Arbella in a War of the Scottish Succession?