Well, there won't be new materials for Corona 12 as such. There will be two things - the Corona Material Library materials inside Cosmos, but those will be the same as you had from the Corona Material Library, so not new :) As an aside, the extra ones that have accidentally showed up and gone again are the V-Ray conversions of the Corona Material Library materials, so these are not extras that V-Ray gets and we don't.
And then there will be some brand new materials, in the next Cosmos update, which will happen before the release of 12 if things go to plan, so you won't need to wait for Corona 12 to see those :)