Sure technically it was open, but in reality only a few houses were realistic candidates. The only period, which was more open, was the period during and just after the Great Interregnum. The counts of Nassau and Holland also were elected and even Richard of Cornwall and Alfonso of Castille were elected at one point. However the Electors soon realized, that non native candidates, without a powerbase in the Empire was a mistake.
Moreover successful new dynasties with Rudolf of Habsburg and Henry of Luxembourg (with his son John of Luxembourg) managed to establish themselves amongst the other great houses in the Empire. Indeed Adolf of Nassau, which to do the same thing, when he made his move on Thuringia (and Meissen).
Another option could be a surviving and more imperial orientated house of Valois-Burgundy, or much earlier the (younger) house of Welf, though that would require them to keep most of Bavaria and Saxony. Bavaria lost Austria, Styria and Tyrol (technically also the lands of the house of Andechs and Merania), Saxony OTOH was broken a bit more drastic, perhaps a Saxony, which is broken up more like Bavaria might help a bit.
The house of Wittelsbach is an obvious choice, I'm a lot more sceptic about the real chance of king Francis of France; he was useful to drive up the bribes for the Prince-Electors, though a member from a German dynasty seems like a more obvious choice, if they don't go for Charles V, as IOTL.