She's definitely going to need to remarry. Louis XI tried to force Mary's hand following the death of her father, and I imagine he may try again if Maximilian dies in 1482—though it really depends on how things are going in France—his vassals may not be keen to go off on another adventure for the Burgundian inheritance: an earlier Mad War if he decides to make another push at it?
Sigismund of Tyrol is unmarried in 1482—but I don't really see Frederick III pressing Mary to make the match. His marriage to Catherine of Saxony in 1484 was pretty much regarded as a farce: he was considered senile at the time. Plus, it'd be in Frederick's interest to keep his Tyrolean cousin without any issue in hopes to reunifying the Habsburg domains in Austria. Sure, the House of Habsburg is reduced with Maximilian's death to Frederick, Sigismund, and the young Philip, but I can't see Frederick III pressing the match... or Mary seriously considering it. She's in the flush of her youth with the Lowlands as a potential prize. She can have almost any man in the Christendom.
I could see Frederick III suggesting Philip be sent to Vienna to be reared and raised, though—Frederick would look upon his grandson as his future successor, and there's no age requirement for the King of the Romans. I can seeing him wanting to do his best to control how Philip is reared.
I feel like René, the Duke of Lorraine would be the best match, given their realms are astride—though given the time period, this isn't a requirement. But it'd make administration easier. The Duke of Brittany could be fun if she can hold off marrying someone else ASAP, but it could be a potential headache—would she go to Brittany, or would he come to her? If she leaves, she's going to need a Regent, but there is Margaret of York, of course. Assuming things in England carry on and Mary remains unmarried for a bit, there could be a suggestion that Mary marry Richard upon the death of Anne Neville? That'd certainly be interesting, especially given French support of Henry Tudor.
Another fun idea would be Frederick IV of Naples. There were plans in the 1470s for a possible marriage between the two, but they ultimately came to naught and he married Anne of Savoy. She died in 1480, though, and he didn't marry his second wife until 1487. There is not yet any hint that France might tear the Italian peninsula asunder, and Frederick IV c. 1482 has no male heir—Mary would have the chance to become Queen of Naples and to possibly sire a future king, too.
There's no rush for Mary to find a second husband, but given the political situation, I think it'll happen at some point. She has a son and a daughter, but child mortality in the period means there's no guarantees—either child could be whisked off at the slightest fall or cough. She was the recognized heiress, of course, but it was pretty much expected in this period of history that a woman needed a husband beside to rule—and most heiresses in this period did marry and reign with their spouses.