Let's clarify what they've said in the past:
--They will never do an annual release
--They won't do any more "bubble plots" or side games like Halo Wars or ODST because they want everything to tie into the Reclaimer Saga.
--They shot down Halo 2: Anniversary a few weeks ago.
--Alongside shooting down a Spops Season 2 at about the same time.
--They already confirmed that they were working on the next Halo game since before Halo 4's release.
--They also confirmed that they have projects on the Xbox One, including the TV show I imagine.
--BS Angel dropped a huge reference to Halo 5 in last week's bulletin, saying that whenever she peers over at her colleague's monitor, she sees either Spartan Assault or an as of yet unannounced project, with a massive NO COMMENT to emphasize the point.
--We know 343i is going to be at E3 in some capacity as its promoting Spartan Assault.
--We also know that MS is going to be talking about their 15+ games at E3 currently in development for the X1, and Halo 5 has already been confirmed to be in development.
--We also know that if it's announced at E3 this year it'll be released next year, making for an at least 2 and a half year dev cycle, with the largest team in the franchise's history. Enough time I'd say to make a full game.
If 343i has anything at E3, it'll be Halo 5.