There is very little we know about Halo Infinite right now, but one of the things that was conclusively and concretely established at E3 was that it will be made on a brand new engine, the SlipSpace Engine. In fact, one can argue that the Halo Infinite reveal was as much of one for the engine as it was for the game.
That engine may actually still be under development. 343 Industries’ job listings still reference the engine as being currently under development- although, there is always the possibility that the listings have simply not been updated, which does seem unlikely, since they refer to Halo Infinite by its name, so it had to have gone up after the game’s launch. Alternatively, maybe they are referencing the development in present tense simply because it is easier to attract and excite a prospective applicant by telling them something is being done, rather than something was done.
“Are you a talented engineer obsessed with developing incredible game engine technology?” the listing reads, looking for a Software Development Engineer. “Are you passionate about building performance critical systems to enable content creators to deliver world class experiences? Are you familiar with low level engine systems such as the scheduler, memory manager, and binary layout of game runtime data?
“343 Industries, the studio developing Halo Infinite, is looking for a talented, passionate, and effective Software Development Engineer to join our core engine team building the SlipSpace engine to power Halo Infinite. Come be part of a studio that pushes boundaries of technology, creativity and imagination. A studio that is player obsessed. A place where teammates seek to amplify each other.”
If the engine still is being developed, however, it may actually end up being a while before we get to see Halo Infinite. Some rumors had pegged it for a 2019 release, at least for the single player portion- but even if the engine is not complete right now, I don’t see how that could possibly happen.