I have created a draft of the data structure which will be used in the Soundfont Format version 3. Please check it out. It is the latest issue which I have posted on the github page.
Whatever you do, don’t name it V3 or SF3 because SF3 is already in existence and allows for Ogg containers (currently specified for the Vorbis codec only, but Ogg FLAC would be a possibility) in an otherwise SF2-alike (with minor differences) soundfont.
SFZ is completely different, too, so it’s not that. It’s a different standard that can coëxist.
I’m not so sure we need a(nother)/new format, either…