(Before I clicked "Add answer," I re-read this and need to say, in advance, that some of it is a general rant and in no way directed at you, Sue.)
I think the idea would be great in theory, Sue, but I'm not certain it's practical. The whole GDPR mess aside--which, trust me, I very much wish I could toss aside--it's extremely difficult to work backward from DNA only using a "one-tree" like WikiTree. It can and is often done using private trees, or on "mirror" trees on sites like Ancestry, but the problem here--my opinion only--is adding multiple profiles that are only speculative would cause more confusion than it could correct, and that would fly in the face of WikiTree mission statement: "Our mission is to grow an accurate single family tree..."
Neither yDNA nor mtDNA can be used to predict recent-generation relationships, and autosomal DNA without paper-trail corroboration can really only do that at the level of parent/child and possibly full sibling. The average amount of expected shared DNA with a half-sibling, aunt/uncle, or niece/nephew is identical, so DNA as predictive evidence is already beginning to break down at that point...and the variability of the amount of shared DNA becomes more random and more pronounced the more removed is the relationship. This is also all assuming the family tree has no pedigree collapse, which every tree in the world does at some point.
There's seemingly a general assumption that DNA evidence is pretty straightforward to use, that in every case it's binary, either option A or option B. But it isn't. In truth--and I'm not going out on much of a limb here, scientifically speaking--there's a good chance that a casually constructed atDNA triangulation confirming 4th cousins to shared 3g-grandparents that includes a paper-trail is going to be indefensible if closely examined. It's very difficult to be accurate with DNA evidence even in conjunction with solid traditional paper-trail as far out as 4th cousins. Third cousins aren't always the slam-dunk that we accept here on WikiTree, but a line had to be drawn somewhere. To use be able to use DNA evidence accurately out to 5th cousins is, in fact, highly unlikely and I would go so far as to say that vast majority of "confirmed with DNA" settings you see out that far would be erroneous if put under a microscope.
I get slammed--sometimes playfully, sometimes not--for posts that are way too wordy. Guilty as charged. But the fundamental problem is that accurately using DNA in genealogy is not simple. And trying to squish explanations of very complex subjects down to a 100-word flashcard is a recipe for disaster.
Kind of like needing to perform an emergency appendectomy out in the field and telling the doctor on the phone to quit talking so much and just tell you where to cut.
Heck, I know people who self-style themselves as professional genetic genealogists who can't, without cheating, define ROH or recLOH or heteroplasmy, much less explain why they're important to consider in genealogy. The bottom line is, if you want to use atDNA out to 2nd and probably 3rd cousins, you're working with the first-aid kit and are probably good to go. But working past 3rd cousins is performing surgery without the training.
Ahem. Sorry. Big soapbox issue that comes up for me all the time. What we've been seeing the past few years with DNA is not dissimilar to what we saw when the World Wide Web opened up online family trees to the wholesale merging and subsequent propagation of false information. Today, someone posts "DNA proof" of a set of relationships, and the next person comes along and accepts it as fact, no questions asked. But much of the time it is, just like that unexamined online family tree, false.
Soapbox packed away for another day. Cough, cough.
I think the heart of the question is that hypothetical DNA matches can be tremendously valuable and need to be researched and analyzed...exactly what our Adoption Angels do every day. But that's probably best done away from the one-tree itself. Someone pops up as a 15cM match, I can guarantee that's going to take some work to determine if it's genealogically valid, much less how it might accurately fit into a family tree. It isn't enough to simply do a broad brush on DNA Painter and believe the line of segment inheritance is determined because it looks like it fits (or doesn't fit) somewhere among the grandparents. If the goal is accuracy, that is.
Maybe a working FreeSpace page, reference to which can be included in a "=== Research Notes ===" section on the various profiles in that particular relationship chain. That way cousins looking at the profile know what's up, and might even be able to contribute to the collaborative analysis happening on the Space page.
I'll close by apologizing again, Sue. Folks who read this far are the ones truly interested in accuracy in their research. The ones who bailed early just want to know where to cut to get at the stupid appendix. :-)