UPDATE 26/03/2014 6.56pm: The first trailer for Halo Online has been released. It looks a little something like this:
As you can see at the 1.27 mark, there appears to be micro-transactions where you can purchase equipment with in-game currency, or purchase in-game currency with real money. As such, it seems to have adopted the contentious "pay-to-win" model.
According to a translation on Reddit, a couple of the subtitles read, "Create a unique Spartan" and "Select original weapons and armour."
ORIGINAL STORY 25/03/2014 9.10pm: Halo is getting its own free-to-play spin-off with Halo Online, a game going into closed beta this spring exclusively in Russia.
The series' current handlers at 343 Industries noted on Halo Waypoint that Halo Online is being developed with Saber Interactive (who helped on the Halo: Master Chief Collection Halo: Combat Evolved Anniversary) and Russian publisher Innova Systems.
Halo Online will run on a highly modified version of the Halo 3 engine. It won't have a single-player campaign as the entire game will be set on a Spartan training facility aboard a secret UNSC space installation called Anvil.
When asked about coming to other territories, 343 said in its FAQ, "Right now our focus is on learning as much as we can from the closed beta period in Russia. Theoretically, any expansion outside of Russia would have to go through region-specific changes to address player expectations."
As far as other platforms are concerned, the developer noted that Halo Online "was built from the ground up specifically for the PC. There are no plans to bring Halo Online to the Xbox One."
No release window has been announced at this time.