I confess that Edmond Rostand's Cyrano de Bergerac is one of my favorite works of literature and Cyrano is one of my favorite literary characters, so I care very much about what happens to him both in the story and even beyond as we consider another act for the play.
By the end of the play, all of the characters in the play are still alive except for Cyrano and Christian. Obviously neither of them can come back to life (you said the idea had to be "likely"), so we have to deal with those who are still living.
One approach to this kind of a "sequel" is to resolve or renew some kind of conflict from the original play. The most obvious antagonisms and conflicts were between Cryano and, well, just about everyone. The next greatest conflict is between Roxane and De Guiche, though that relationship seems to have mellowed by the end of the play. Since all the major external conflicts are gone, it may be that the focus should be on the internal conflicts.
Everyone in the play is touched, in one way or another, by Cyrano's death. His friends (primarily Le Bret and Ragueneau) are outraged at the manner in which he died (foul play, of course), but they are not likely to seek revenge for their friend's death. The sisters of the convent are sad about the loss of such a lovely soul, and they will certainly continue to pray for him but can do little else.
That leaves Roxane and De Guiche again. Of course an obvious choice is to have De Guiche and Roxane get together somehow, but he is presumably still married and that seems to be a cheap and easy option.
Instead, it seems to me that both of them have an interest in wanting to ensure that Cyrano's name and legacy of "panache" do not die. Unfortunately, one of Cyrano's final statements will make that quite difficult for them, assuming they accede to his wish:
It's been my life's role to prompt others to greatness and to be forgotten myself.
All of that leads to one theme with many possibilities stemming from it--Roxane and De Guiche must do something to promote the things Cyrano committed himself to during his life. Consider the following:
The arts - Cyrano loved words, poetry, and good drama, so perhaps they somehow support those endeavors in others. A school, a theatre, or some other venue in which poets and writers can learn, grow, and flourish.
The military - Cyrano was a fearless soldier and loved the men of his guard. Perhaps there is something they can do to honor their friend in that area.
Comedy - Cyrano loved to laugh and had a delightful (and incisive) sense of humor. How about a big party or celebration to entertain the people,
These are a little insipid (weak) for these two characters. Though De Guiche was often outwitted by Cyrano, he did his share of maneuvering and trickery. Roxane, after all, manages to visit her husband on the front lines across the country and during a siege. They have a kind of courage which can now be turned against Cyrano's enemies; Roxane and De Guiche can avenge Cyrano in all manner of ways. AND they can be protected, since De Guiche is the Cardinal's nephew.
He has mellowed, but he may as well use his manipulative and vengeful nature to get avenge someone who outwitted him at every turn. Roxane is plenty bright enough to do the same for the man she just discovered she loved. How they avenge Cyrano is up to you. Maybe they do secretive and devious things to destroy his enemies, leaving a feather behind so they know it was for Cyrano.
Please, no giant "nose" statues.