Some recent thoughts:
As with
mainstream supersymmetry, I see the sbootstrap's situation as still being one where there is such a multitude of possibilities that it is hard even to systematically enumerate them. The difference is that a mainstream susy model consists of a definite equation and a resulting parameter space that then gets squeezed by experiment, whereas a sbootstrap "possibility" consists of a list of numerical or structural patterns in known physics which are posited to have a cause, and then an "idea for a model" that could cause them. It may be that some part of sbootstrap lore is eventually realized within a genuinely well-defined model that will then make predictions for MSSM objects like gluinos, or it may be that it will be a "minimalist" model that is more like SM than MSSM. (As for the Koide waterfall, that is such a tight structure, it seems that any rigorous model that can reproduce it is going to be sharply predictive - but there may still be several, or even many, such models.)
Today I want to report just another "idea for a model". It's really just a wacky "what if"; I don't know that such a model exists mathematically; but I'd never even look for it if I didn't have the schematic idea. The idea is just that there might be a brane model in which the top yukawa is close to 1 both in the far UV and in the far IR, and that this is due to a stringy "UV/IR connection".
The reason to think about this is as follows. The discussion of whether the Higgs mass might be in a narrow metastable zone, has
yielded the perspective that it might be worth considering the top yukawa and the Higgs quartic coupling at the same time. The latter goes to 0 in the UV, the latter goes very close to 1 in the IR.
But
Rodejohann and Zhang have observed that with massive neutrinos, the top yukawa can approach 1 at high energies as well (see pages 14 and 15; the minimum is roughly 0.5, reached at about 10^15 GeV). And high energies are where a coupling might naturally take a simple value like 1. So
what if there's a brane model where the top yukawa is 1 in the far UV, for some relatively simple reason, and then it is also near 1 in the far IR, because of a UV/IR relation that we don't understand yet? String theory contains
UV/IR relations (scroll halfway down for the discussion); none of them appear to be immediately applicable to this scenario; but such relations are far from being fully understood.
At the same time, I think of Christopher Hill's recent papers (
1 2, it's basically the same paper twice), in which he first restricts the SM to just the top and the Higgs, and then considers a novel symmetry transformation, which he likens to a degenerate form of susy. At high scales he ends up with the relation that the Higgs quartic equals half of the square of the top yukawa - which is not what I'm looking for. Then again, he also ends up with Higgs mass equals top mass, with the difference to be produced by higher-order corrections. So perhaps his model, already twisted away from ordinary supersymmetry, can be twisted a little further to yield a Rodejohann-Zhang RG flow for the top yukawa, as well as a Shaposhnikov-Wetterich boundary condition for the Higgs quartic.
One might want to see whether this can all be embedded in something like the
"minimal quiver standard model" (MQSM), which is not yet a brane model, but it is a sort of field theory that can arise as the low-energy limit of a brane model; and the MQSM is the simplest quiver model containing the SM.
Finally, to round things out, one might seek to realize the sbootstrap's own deviant "version of susy" here too, perhaps by using one of the brane-based "ideas for a model" already discussed in this thread.