Efneo gearbox is an interesting option, but it only gears up; good for small wheelers, not so good otherwise. Gear ratios are
This is the gear spread with 28T chainring, 20T sprocket, when coupled with a SA 3s hub;http://ritzelrechner.de/?GR=SAAW&KB=50,40,28&RZ=20&UF=2185&TF=90&SL=2.6&UN=MPH&DV=gearInches
Sensible gear range, but from the theoretical 9 ratios, many of them are close duplicates (within ~6%), so effectively only five distinct ratios. And that combination of chainring and sprocket won't fit into that (or perhaps any) chaincase, either.
If the gearbox geared down from a middle ratio and also geared up (by a smaller percentage) then you could end up with gearing like thishttp://ritzelrechner.de/?GR=SAAW&KB=53,46,30&RZ=21&UF=2185&TF=90&SL=2.6&UN=MPH&DV=gearInches
when coupled to a 3s gear, effectively giving a 1-1/2 step down and a 1/2 step up. In this case the chainring (46T) and sprocket (22T) would fit in the chaincase too. This would give gears from 29" to 91" with a 33% interval 1-2 and the remaining intervals of 15%, with eight distinct ratios, only one duplicate.