At the risk of stating the obvious, I think for military use (non-reloadable) it will be driven by cost and weight. Obviously, brass is not good for either of those.
Steel is good for cost, but not so much for weight (not much better than brass). Plastic is much better for weight and might also be very good for cost if a solution can be injection molded (like CT ammo). I have to think that molded CT cases would be a fraction of the cost of a drawn metal case (brass or steel) in serial production. But these still need more development (which costs $$$).
In the interim, I think hybrid cases might become mainstream for military SAA. By hybrid I mean two or more materials combined (metal/polymer like True Velocity or metal/metal like Shell Shock Technologies). This should be the quickest way to save weight since properties can be tailored for each section of the cartridge case without radical changes like CT ammo.
It doesn’t look like SST does any bottle-necked rifle cases and I have not seen any indication that they are pursuing military contracts. I saw that SIG is promoting a steel/brass case for the 6.8, but I don’t see that as much more than a potential cost savings (carbon steel is about 8% less dense than cartridge brass, so only a slight weight saving). It seems like a high-strength aluminum head (like SST) combined with a steel or polymer case body would give the best weight savings without too much development.
One potential roadblock to hybrid cases is separation of the pieces. It will be interesting to see how this plays out under harsh military conditions.