The version of GDT that includes the non-beta Modding API will be 1.5.x, so yes, there will be a 1.5.0, although you may not see it (the public release could easily be 1.5.1 or .2 if there are last minute bugfixes). That will also be when the API will be available for platforms and editions other than the Steam edition for Windows.
Also, it is rare for a developer to release information about their next project until the project is well established (CAUSE: “Our new product is better than our old one.” EFFECT: People stop buying the old product in anticipation of the new one). GDT itself was in development for 2 years before anyone outside of the Brothers Klug and their collaborators heard about it. And it was a further 6-9 months before the wider world heard about the Desktop edition.
So you’ll hear about the next project when Greenheart are ready to release that information.
PS: Steam trading cards are controlled by Valve. Whilst GHG has some say, I doubt Valve will see the need to add any more.