Mind-Machine-Mind Interface (MMMI) is, or would be if it existed, a kind of neural interface. The important distinction is that it uses machine only as the intermediary between two brains, so transferred information do not have to be fully understood, and massive processing power to generate virtual reality is not required.
Depending on the extensiveness of the link, and it's connection points, contact might range from anything to simple communication, through empathy and telepathy, to creation of fully collective mind.
Consequences of MMMI in any form more serious than just data transfer could be very significant. It might lead to create peaceful, cooperating society, where every person understands each other. It may create super being, whose knowledge and power would exceed that of humanity. It may lead to things that today we cannot even imagine.
But what would happen if collective mind would develop within the net? And if many of the basic human traits, like conscience or empathy, did not transfer, becoming lost in the machine? New consciousness would be created - inside all the mind of the collective, yet at the same time external. That might be how the Borg came into existence. This new mind, triggered by random fluctuations in massive neural net of the collective mind would want what every living being wants - to live, and to grow.
In the beginning it would have to only control these machines/people who build MMMI devices, and modify them to suppress thoughts that do not conform to consciousness objectives. In effect it will suppress all free will and initiative. Then it will proceed to assimilate all available resources.
I don't know enough about large scale collective minds, and I doubt that anyone knows that, to know if such possibility exists. And anyway it should be relatively easy to protect from, unless significant portions of individuality was given up already. It should be hoped that if humanity is given that choice, it will create only loosely connected net, so that while everyone will remain part of it, one will remain oneself, and not just a part of massive computational network and mobile manipulator.