Hey, maybe it's time to make a comeback à la Medusa! (I'm not taking 25 years though). I'm glad to see that there's still support for our beloved Gorgon-inspired goddess! And thank you a lot WhiteEaglePL for reusing my previous thread, I love the changes as well.
Now, about Medusa's chances for Ultimate... I'll admit that I think it's low. The number of newcomers will be limited this time afterall. This and the already overwhelming content in Smash 4 and probably Ultimate (I'm glad to see that we will get the Reset Bomb Forest in HD!). Kid Icarus also didn't get any new content since Smash 4.
On the other hand, three things can save her: importance, originality and the good old Sakurai bias. Originality is the most important in my opinion. We can see that since Smash 4, especially the DLCs, Sakurai is looking for characters that can bring things new to the table. Ryu, Bayonetta, and most importantly Corrin, that was added despite the "too much Fire Emblem" problem. Inklings and Ridley stay true to this as well, although it's also a matter of popularity. Medusa still have a lot to become a unique fighter in many ways.
Now I hope that Sakurai still love his "role" thing from Brawl's GDC 2008 presentation. I believe that these still exist for the new characters (Ridley being way more monstruous than any other fighter for example), so having Medusa as the female villain is still a plus.