There are various curb-less shower systems out there. Some require major restructuring in a retrofit, but newer ones do not. New systems incorporate a single slope, which is an easier install than the old shallow cone shape, and often have linear drains at the back of the shower. Because you have a small space, you might consider using some of the concepts of a "wet room," which would make water splashing around no big deal. See Houzz under "Trend Accessibility." You are not alone in thinking ahead.

You need a 2'' height difference between the entry to a shower and the drain. Plus the slope can only be between 1/4"&1/2" per foot. That translates to the fact that a curbless shower has to be 4' or larger, with the drain located at the far wall from the entrance to satisfy both requirements. Or, the entire room can be sloped towards the drain as a wetroom, but that can lead to higher thresholds at the entrance that can create a barrier there.

Maybe you could go ahead and get this done, and then maybe take up yoga-- which might keep you forever from having to use a walker. :)