In Halo: Cryptum, the Forerunner Capital was composed of massive, circular “platforms” that housed cities. In the event of an attack, the platforms could rotate and form a sphere, enclosing the cities inside a protective shell. The planet shown in the teaser could be the Forerunner capital city.
In this video, at 8:00ish Frankie states that the “planet” is an artificial construct; this fits the description of the Forerunner capital.
It IS just a guess. I went at it from the standpoint of what would make sense within the franchise: something already mentioned, not something from out of left field that might alienate fans. The only other spherical constructs we know of are the Onyx Dyson sphere (debunked) and another shield world. Shield worlds have been done already, so a non-shield world construct would be newer and more interesting. If John is to face his “destiny” and (likely) encounter the Forerunners, the Capital is one of the most convenient places to do so - it’s full of Forerunner stuff. The Capital surviving the Forerunner-Flood War is a difficult problem to look past, but it survived the first attack, and the Forerunner are able to move large constructs like Halo rings, so the Capital could have been moved outside the galaxy, possibly near the Ark, to protect it from the Flood and Halo. Moving large objects like Halo rings and worlds through slipspace causes massive stress on their structure, so it might be difficult or unlikely to transport the Capital. However, by the end of Cryptum, the attack on the Capital left the Ecumene’s government in tatters - hardly in a state to construct another shield world. While it is possible the planet shown by the teaser is a shield world built before the attack on the Capital, the Capital was not destroyed, but remained vulnerable in the Orion complex. While it would cause further damage, I think it would be a smaller loss to preserve what was left of the Capital by transporting it rather than leave it vulnerable as a senseless waste of resources.