POSSIBLE SPOILERS!! POSSIBLE SPOILERS!! I HAVE AN OPINION!! HIGHLIGHT IF YOU WANT TO SEE!!
The Legendary ending is just an extra 10 seconds...it's nice...but very short and leaves you wondering what's going to happen next. I believe it could possibly be a re-make of Marathon since that game took place 200 years after Halo. Bungie did say it was the end of the Halo series, but that doesn't mean Master Chief. The future series (Marathon, possibly Forerunner) could still have John 117 as the main character, but there probably won't be any Halos anymore...a new story line.
I really hope it's Marathon though...it took place in the year 2794, around 200 years after the Halo Series (2500)...200 years seems a long way away from the Halo series, but Cortana does say it could be a while for someone to find their beacon, possibly years. It would be great to play as Master Chief again (John 117) but on a different story that's not about Halo, but something different. On the Legendary ending, there was a planet with lights, that could possibly be Onyx (in the novels) or another planet related to Forerunner. Anyway, I hope the next game is with Master Chief, whether it be Marathon, Forerunner, or another story line...based on the ending, there's a high chance of it happening.
END OF POSSIBLE SPOILERS!! END OF MY OPINION!!
When you finish the game, on any difficulty, wait after the credits for more...