I don't think much will happen on the size of a marker any more because there is the physical constraints of the human body to deal with.....to small a gun is unuseable.
Weight? my other passion is mountainbikes and in their evolution they just got lighter and lighter, but then they started to break because they were simply not strong enough, but now as better materials are being used the weights have started to fall, but "NOT" dramatically. Every Gram shaved off now is by endless hours of engineering trickery. Markers will still fall in weight for a while but when you bring it down to it's basics of a thin walled piece of tubing, it looks damn ugly and no-one will buy it, so i'm affraid weight will stay "up" in order that the marker looks good.
Hoppers and Bottles? That's where the next weight saving gains will be made, and what about a lighter paintball that somehow still flies as well?
Consistancy? Better reg improvements etc etc to put a true ball on ball consistancy. How about electronic on the fly velocity adjustment that will maintain 299fps all day, hot or cold? I reckon the technology already exsists for that one
Reliability? The maintenance free marker?
Efficiency? Be honest, if you filled up before the game have you ever run out? not really an issue then is it?
Cost? I'm not saying it is good but the epihany looks good and at a good price, same goes for the Rail. Good looking reliable markers that shoot with the best of them....the way foreward
Of course all that sounds really boring so i'll stick with my cocker