I believe that it's going to be a legendary. The devs are aware of the recent anger towards too many marquees, and Revan is too big for that kind anyway. Hero's Journey events always included a previous version of the toon you're getting (Farmboy to CLS, Scavenger to JTR), and there's no version of Revan. Plus, the release video started with the words "a legend from the old republic has emerged", which again points to legendary specifically.
All that being said, they might have created a new type of event, or put him in some existing structure. I really don't know... But my guess is that I'll know very soon.
