"Your future graduation."
Translation:Η μελλοντική σου αποφοίτηση.
Archaic? Not so, my friend! I am not sure of your age or your place of residence, but I know for a fact that this form, μέλλουσα, is very much in use today.
I personally live in Greece and as a native, I haven't heard the word μέλλουσα in a while :P It's mostly used in cases like "μέλλουσα σύζυγος/νύφη/γυναίκα, μέλλουσα μητέρα/μαμά, μέλλουσα κυρία (insert a surname)", etc. It's more commonly (if not exclusively) used for people, rather than 'situations' (like αποφοίτηση). ^.^
To be more precise: Mέλλουσα is the fem. past participle of the Ancient verb μέλλω: Participle: μέλλων-μέλλουσα-μέλλον. Some dictionaries as this one: https://el.wiktionary.org/wiki/%CE%BC%CE%AD%CE%BB%CE%BB%CF%89%CE%BD, consider it as an adjective. From this participle we can say comes the noun μέλλον=future.
Μελλοντικός/ή/ο= A pure adjective, that we can say it is synonymous with the above.
We can say they are interchangeable. But look about the phrase: Μελλοντική αποφοίτηση and μέλλουσα αποφοίτηση. They are graduations that they will take part in the future. It seems to me the second one more archaic and formal, even it is not much.
Some expressions like μέλλουσα ζωή=Afterlife, are unique, Ancient Greek, religious and Church's and we cannot replace it with the phrase μελλοντική ζωή, it is something different, general, it can be used in many circumstances, including graduation, not for Afterlife.