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