This leaker claims that Halo: Infinite will have 25 missions and will take around 20-hours to complete the campaign. Additionally, there will be 35 side missions which add an additional 15 hours of gaming. During the main campaign players will be Master Chief (not really a surprise as this has been largely confirmed by 343i). Other characters supposedly returning are: Arbiter, Atriox, Locke and Palmer.
The leaker goes on to go over additional rather juicy story details. As we know Halo 5: Guardians ended with Cortana going rogue, launching a plan to destroy everything. Allegedly Master Chief captures Cortana and somehow sends her back in time to stop the galactic civil war from happening. All of this ends the campaign on the planet of Reach where this time travel plot actually ends up rebooting the Halo series.
With any leaks there’s certainly A LOT of room for error or trolls, but this does seem certainly possible.