Well, a trans doesn't HAVE to engage all the gears at once. I recall making something like this from my lego days, might need computer shift assist to not break teeth, but more efficient design anyway. If it is on a bike (or car) with chain reduction then some way to have direct drive in high gear with no meshing would also be beneficial.
(found this messing around with google sketch) envision the output on a sliding splined shaft.