What could be some teachings that Buddha has not revealed to us?

The teachings to be passed on by Maitreya.

As for the teachings of Maitreya Buddha, they are yet to be seen.

Do we have any knowledge about the teaching that he has not revealed to us?

There are three factors to determine what the Buddha has taught:

One is, the teaching is not limited by what the teacher can teach, but by what the student is capable of receiving. An Einstein, when being the teacher for a kindergarten, he can only teach ABC.

Two, the Buddha only responses to question. In general, the Buddha doesn't teach if not requested. Therefore, how good our classmates were able to ask the question determine what we could have in the Sutras. Example, since Ananda didn't ask the Buddha to clarify what were the minor precepts that could be removed, it indirectly caused the spilt of Samgha after the Nirvana.

Last is, some of the sublime knowledge are revealed when a practitioner reached advanced stage. By that time, the Buddha or Bodhisattva will teach the practitioner directly, as stated in some Chinese Sutras.

Could that be the teaching of tantra?

From the Original Mahayana school, which is the Chinese lineage, Tantra is categorized as non-Buddhist. By the Tang Dynasty (600CE), Buddhism in India started to mix with Hinduism, the old Indic religion in the form of Shaivism reared its head again and turned Buddhism to its host. Three famous "Buddhists" came to China and translated their "Tantric Sutras" into Chinese, they received welcoming receptions from the Emperor. However, once their texts came out the Emperor with the learnt Buddhist masters and scholars reviewed, he confiscated the "Tantric Sutras" and ordered that these texts should never be circulated because it was against the Buddhist principles, and the moral degradation and corruption embedded in them. The Emperor bounteously rewarded and gifted the three "Buddhists" then sent them away. Until 700-800 years later, in 1300CE, when China fell to the hands of the Mongolians, the Mongolian kings were followers of Mongolian Shamanism and favoured "Tantric Buddhism". These texts once were confiscated by the Emperors of the Hans were promoted and started to gain popularity. _{(Chinese were/are fetish of keeping knowledge bits and historical bits, they will never destroy any bits of knowledge but faithfully keeping all evidences.)}

Buddhism in Tibet was first transmitted from China. In 650CE, Princess Wencheng bought the Chinese Sutras and translated to Tibetan. Until Moheyan (800CE) lost the debate to Kamalaśīla who represented Indian Buddhism, Tibetan Buddhism turned fully Tantric, mixed with Bon and empowered by Padmasambhāva (800CE), followed by many Rinpoches coming from India to transmit the Vajrayana | Tibetan | Tantra "Buddhism". In fact, only Tibetan Buddhism itself and the Rinpoches declared themselves the "Mahayana" school, as of today, 1200 years after Moheyan the Mahayana Buddhist was rejected and expelled from Tibet.

I just recount the history of Tantra the part related to Buddhism, how would you like to take it is your call.

In the Mahaparinibbana Sutta, the Buddha said he kept nothing hidden from his disciples.

Yet in the Simsapa Sutta, the Buddha said he knew more than what he taught.

So what didn't he teach?

He did not teach anything that was not useful towards the goal of ending suffering, as you can see in the Parable of the Poisoned Arrow below.

The Parable of the Poisoned Arrow from Cula-Malunkyovada Sutta:

"It's just as if a man were wounded with an arrow thickly smeared with poison. His friends & companions, kinsmen & relatives would provide him with a surgeon, and the man would say, 'I won't have this arrow removed until I know whether the man who wounded me was a noble warrior, a brahman, a merchant, or a worker.' He would say, 'I won't have this arrow removed until I know the given name & clan name of the man who wounded me... until I know whether he was tall, medium, or short... until I know whether he was dark, ruddy-brown, or golden-colored... until I know his home village, town, or city... until I know whether the bow with which I was wounded was a long bow or a crossbow... until I know whether the bowstring with which I was wounded was fiber, bamboo threads, sinew, hemp, or bark... until I know whether the shaft with which I was wounded was wild or cultivated... until I know whether the feathers of the shaft with which I was wounded were those of a vulture, a stork, a hawk, a peac**k, or another bird... until I know whether the shaft with which I was wounded was bound with the sinew of an ox, a water buffalo, a langur, or a monkey.' He would say, 'I won't have this arrow removed until I know whether the shaft with which I was wounded was that of a common arrow, a curved arrow, a barbed, a calf-toothed, or an oleander arrow.' The man would die and those things would still remain unknown to him.

"In the same way, if anyone were to say, 'I won't live the holy life under the Blessed One as long as he does not declare to me that 'The cosmos is eternal,'... or that 'After death a Tathagata neither exists nor does not exist,' the man would die and those things would still remain undeclared by the Tathagata.

A simple example illustrates the Buddhas vast knowledge.

The Buddha was Sakka thirty-six times, which presumably gave him skills and knowledge beyond our ken. Yet I also very much doubt that teaching us how to be Sakka would be of any use to us here and now in our journey along the Noble Eightfold Path:

AN7.62:2.2: I was Sakka, lord of gods, thirty-six times.

A more concrete example is DN1, which summarizes all the ways (i.e., teachings) where one can get "lost in the weeds" thinking about the past and the future. These are comprehensive yet not fully elaborated, since elaborating wrong views would be of no use to those with right view. Summarizing suffices to hold us back from wasting time pursuing such teachings:

DN1:3.72.1: All of these ascetics and brahmins who theorize about the past or the future are trapped in the net of these sixty-two grounds, so that wherever they emerge they are caught and trapped in this very net.

The suttas dedicate themselves to the end of suffering. Other teachings are a distraction.

The Buddha revealed enough to enable us to discover the rest for ourselves and any practitioner will end up knowing things they have not read in the sutras. After all, in the end we don't need books.

For me the most interesting omission is an explanation of metaphysics (except by implication), but as someone who writes about this topic I can easily understand why he didn't want to do this. His concern was soteriology and not time-consuming theoretical matters. It is said that in his time there was a prediction that someone would coma along later to explain metaphysics, and a few centuries later Nagarjuna does so in great detail.

It would be implausible to suppose that the Buddha said all he had to say given his understanding of Consciousness and Reality, but really there would have been no point is saying much more since being told is no substitute for discovery and knowledge.

I'd suggest he mentioned everything he needed to mention of the sake of our salvation and anything else was not worth mentioning because we will discover it anyway if we take the medicine and follow the method, while being told about it will not help us.

As for Mahayana and Theosophy, I don't think they need the justification of pointing to what the Buddha did not mention. If no sutras existed they would still need to justify themselves. Indeed, it may be the other way around since Nagarjuna's metaphysics is a philosophical justification for the sutra teachings.

Not sure that this is really an answer but perhaps its relevant.

Yes, of-course buddha hid lot of teachings. They were mostly related to superpowers like how to use capability of brain to control others, flying, instant-transmission, preaching to deva-lokas, teaching in future(time travel) and what not?

Alas! It was all for nothing as later on various tantriks at nalanda university were probably having advanced knowledge on all this and for goodness, it was burnt down otherwise today would have been worse in terms of black magic, sex-tantrism, immoral monks and much more.

These type of teachings were never required and have nothing to do with attaining nibbana. 😠