Prometheans were created to guard the galaxy and destroy the flood.

Spartans were intended to crush human rebellion but were found to be the most effect weapon against all things. (maybe even guarding the galaxy)

Prometheans are made from a persons (life force?)

Spartans are made from a person aswell

Both start as normal specimen of their species and are then mutilated into killing machines with no emotion and with one purpose of getting the mission done. (with the exception of the spartan candidates being genetically abnormal from a usual human)

They both have a person stuck inside of them is what im getting at

Mark My Words, There will be links of Chief trying to find his humanity and Promethean being turned back into their original species. It can go one of two ways, Chief will either think he is a killing machine and nothing more (fight till he dies maybe) or he will try and find his humanity and pursue it. For the prometheans, they cant be turned back into their normal species and will forever be a killing machine or a possible way of exposing the prometheans (life force) and either turning it back into its original species or some sort of way of communicating with its original form.