WI: Mary I had been married off before she succeeded the English throne?

Exactly as it says on the tin. What if old Harry decides to marry off his daughter with the condition that if Edward dies without an heir, Mary will be sole ruler. Or what if she was married before the Great Matter happened?

HVIII liked playing games with his children's betrothals, and until he had a boy there was a very real threat Mary would carry England into a foreign empire.

That said, if it's before the Great Matter, Mary was betrothed to the duc de Bretagne, Karl V, the duc again, a prospective match with James V and then the duke of Beja.

After the King's remarriage, Hal's even more loathe to marry her off since anyone who marries her is likely to invade England on her behalf. A good option might be her cousin, Edward Courtenay, as he is English and Plantagenet distaff. Ironically, it was during Karl V's trying to get an alliance with Henry that he offered the infant Felipe II fof the infant Elizabeth I.

Also, Mary made it difficult when she was queen and wished to marry, since she pointed out kings never married below the rank by marrying a subject, thus as queen she would not.

So an ideal prince would be someone who won't inherit jack, and who can come to live in England. Or an Englishman, though I can't see Henry liking that (or even allowing it).