NeoGAF member "KupoNut" discovered some interesting data in the source code of a page announcing the Final Fantasy XV live event called "Uncovered", which will take place on 30th March, 2016. It seems that Square Enix could be announcing a PC port at the event.
He went on to add, "Additionally there's some commented code hinting at more pages that will get added to the page, including a Trailer, Schedule, FAQ, Livestream, What/About and Travel page. The CSS file also has all the styling info for these pages, the pages themselves just aren't live yet."
This rumor seems pretty solid, as plenty of Final Fantasy ports have been making their debut on Steam lately. It seems odd they wouldn't also have Final Fantasy XV on PC too.