Thanks for this posting.
I've been making this suggestion about every year or two for about 10 years. Since FTM seems to want a beginner to "middle-of-the-road" piece of software to serve as the gateway to selling subscriptions, I don't hold much hope for them implementing any structure for documenting relationships. That would put FTM into the forefront of leading edge programs. I don't think FTM will want to scare off beginners with complications.
I heard a few years that the Master Genealogist had some elements of their structure that dealt with it. I recall looking at it several years ago and was not impressed with their implementation.
I don't anticipate that any software will implement just because capabilities are built into Gedcom. I look on Gedcom as a standard that served its purpose in the beginning (1990s) of genealogy software - but most genealogy software providers are now only interested in building in features that no other software has and to get a marketing advantage - not to actually serve the needs of the users.
BTW, a feature of such a structure might include a judgement about the degree of confidence of relationships - similar to the radio buttons for sources:
Proven by DNA
Proven beyond any shadow of doubt
Probably - likely
Possibly - maybe
I would also like facts attached to people on a contingent basis, preceded by the word "possibly" in a Register Report.
James was born... married.... and was possibly the James Smith who enlisted 01/01/1816, in the Civil War, Co A, 99th NY Inf.