I would love a fanfic with this premise. Some weird fusion of the UBW/HF routes where everyone survives and then get drafted into Chaldea (maybe because Marisbilly and Solomon were there instead of Atriam and Medea).
That is not all that shocking, since the uniform were build around Shirou and Rin (genderbent, but still) in mind.
To be more precise, the Gudas were designed as basically genderbent versions of Rin and Shirou, which is why the former work so well as hypothetical children of the latter.