The following contains major spoilers for Moon Knight Episode 3 “The Friendly Type,” streaming now on Disney+.
Steven Grant, in addition to being one of the personas representing the current Avatar of Khonshu, is also a budding expert in Egyptology. During the first episode of Moon Knight, he mentioned that there were two gods missing from the Ennead banners and merchandise in the museum. Traditionally the principal Ennead was the Great Ennead of Heliopolis. This was headed by the sun god and creator Re or Re-Atum or Ra, followed by Shu and Tefnut, deities of air and moisture; Geb and Nut, who represented earth and sky; and Osiris, Isis, Seth, and Nephthys. Osiris was the god of the fields, death and resurrection. Isis was the goddess of magical healing, wifely womanhood and the Queen of Queens. Seth, or Set, was god of storms, disorder and warfare. Nephthys was the goddess of air and mistress of the castle.
Of those nine, Osiris, Isis and Tefnut are represented in the Marvel Cinematic Universe. They have thus far been joined by two others. Hathor, the goddess of music and love, and Horus, who among many other overlapping portfolios, was the god of protection, especially as it pertained to the Egyptian people. That leaves four missing deities unless Khonshu and Ammit are counted among their number, which would be ahistorical, but that is already the case no matter the identities of the remaining gods. This would leave two more unaccounted for which invites the question, who might they be and how might they affect the remaining episodes in the limited series.
As far as where they might be, the third episode of Moon Knight may have provided an answer as somewhere known as the Overvoid. Khonshu described it as opulent and his inference is that the gods live there as absentee landlords who have abdicated their obligations to the mortal realm, lazing about in their own divine decadence. Arthur Harrow, the former Avatar of Khonshu and present Avatar of Ammit, seemed to agree with this assessment since he referred to them as indolent in the series' first episode. In the comics the Overvoid is more commonly referred to as Heliopolis, a dimension similar to Asgard which exists on a vast plateau and can be accessed by a bridge tethered to the mortal world and specifically anchored in Egypt.
In one of the more recent continuities concerning Moon Knight, all the gods, except for Set, were limited to the confines of their home dimension. Their only access to Earth was via hosts they projected their will into and Marc Spector, the primary persona of Moon Knight, was chosen by Khonshu because he had a fractured mind that allowed him easy infiltration given the distance between realms. This differs from the show's paradigm as those gods have chosen to isolate in the Overvoid and can seemingly enter the world of humanity whenever they wish, much as Khonshu has done, but decided that their involvement in the tidings of mortals should be minimal.
In the comics the Ennead is a catch-all term that includes all the Egyptian gods that may show up in its pages and is in no way limited to just nine, so the array of deities that could potentially be identified as those missing from the show are vast if one is accounting for the source material. There are a few that have a particular harmony with existing MCU canon or who are so prominent in the comics that their omission would be unexpected. Set for instance has taken on the veneer of an evil god who has fulfilled the role of chief antagonist in the past to such a degree that his absence would be noteworthy.
Bast the panther goddess was originally a member of the comic's Ennead who later carved out her own dimensional realm in which to reside and was worshipped in Wakanda, becoming their matron goddess and Lady Protector. Since she is already firmly connected to the MCU, as well as playing a role in Thor: Love and Thunder, her inclusion by way of exclusion would make a lot of narrative sense. One of the incongruities that would present themselves is the fact that Steven noticed that there should be nine gods in the museum's materials instead of seven, and it would stand to reason that Bast extricated herself in ancient times, coinciding with the founding of Wakanda. There would be no expectation of her kinship with the Ennead.
This also calls into question the very nature of the museum. If this observation by Steven is meant as some type of foreshadowing then it would be reasonable to consider why the museum's marketing materials would be incomplete in some way. There are well sourced theories that expound on the idea, but it may simply indicate another incidence of Steven displaying his attention to detail and his eventual connection to the detective driven Mr. Knight alter. If Occam's razor is the guiding principle of determining who the missing gods are from the poster then audiences could settle on the idea that they are Khonshu and Ammit, one banished and one dead.
That would still leave two gods unaccounted for that have yet to make an appearance. One of them might be Ra who is often portrayed as an analog to Zeus or Odin, an all-father divinity who holds more authority than the others and therefore may be consulted in only the most dire situations. This would increase the likelihood of that last mystery god being Set, since he could play the diametric counterbalance to Ra, an evil entity that is only included in council when the balance of cosmic power is put in jeopardy or in question. The possibility also exists that Steven's comments are a red herring and hold no particular import, but given the MCU's history of capitalizing on offhand mysteries, it seems highly unlikely.
To see some vacant godly roster spots, Moon Knight is streaming now on Disney+, new episodes airing on Wednesdays.