Please note that "contract" is also synonymous with "civilization", Zarathushtra's main preoccupation.
It was the histroical introduction of the contract as social tool that made Civilization possible. Civilization as in permanent settlements economically based on the DOMESTICATION of animals and plants for food production. This enabled ownership of land (contractual) and the creation of a rule of law (contractual). Without contracts, no civilization!
So Mithras is the divinity of Civilization itself. As Parviz has discussed before in the use of the bersam (fasces) to bind a society together.
Therefore Mithras is also Mind in relation to Being (Culture in relation to Nature).
Each mithraeum apparently had its very own religious practices in what amounted to a completely decentralized religion (as opposed to the extremely centralized and dogmatic Abrahamic faiths).
This sounds like a great idea.
Mithraism is some form has been the popular side of Zoroastrianism since the beginning so let it be the vehicle of Zoroastrian revival today!
I do have my misgivings about Roman Mithraism which had only a partial resemblance to earlier Iranian Mithraism and was possibly a kind of Masonic organisation open to political manipulation.
However there is no reason for modern Mithraists to only use Roman Mithraism as a model - they can go back to Avestan (and possibly Indian) sources as well as well as developing their own ideas of what Mithra means to them.
Probably modern Mithraism will split into lots of different denominations with different ideas. However this need not be a bad thing as long as some of them keep alive the memory of Zoroaster and the Gathas.
So what is Mithra?
Mithra derives from a word meaning contract.
To early Iranian pagans Mithra was a god of justice who stood guard over contracts (agreements made by oaths) and punished those who broke them. Mithra was absolutely upright and fair in his dealings with people though somewhat fierce in punishing wrong-doers.
These pagans were polytheists who worshipped multiple gods, not just Mithra, but it was Mithra who was the most popular.
In the Indian Rig-Veda Mithra is often paired with Varuna who was a sky god with overall concern for good order in the world. To later Indian pagans Mithra (or Mitra or Maitreya) became a more approachable god of friendship .
-- Mithra Today --------
I suggest Mithra should be a deity who guards the 'Good Relationship' (Asha) in all its forms.
Good Relationship between people.
Good Relationship between people and other living beings.
Good Relationship between people and their environment.
Good Relationship between people in their economic activities.
Good Relationship between Mind and Reality.
Mithra guards them all!