GameGuru MAX will support Windows Mixed Reality for compatible Windows 10 systems, yes indeed. The finished standalone executable will detect and launch Windows Mixed Reality portal and headset, so you can play your single player games in VR. There are no plans to support Steam VR at this time, or support multiplayer VR at this time, or support the Windows Store from Microsoft at this time.
The ability to create your level in GameGuru MAX and transfer to AGK may be possible, but GG Loader only supports Classic for now so it would be up to the author of that DLC to add support for MAX in order to get your levels loading inside AGK.
GameGuru Classic will continue to be supported with bug fixes and functionality additions.