I think the next phase in small arms is going to involve polymer cased ammo. Much lighter to pack and potentially a money saver over the long run. I also think that 3D printing will continue to improve. I see this as bigger boon to the military on the front lines producing parts instead of needing to hone the logistics of producing them at a secure facility and transporting them. However, hobbyists may see a lot of use restoring or even printing firearms long out of production. In a perfect world, we could just print what we wanted. However, I could see some sort of vetted licencing with major government restrictions for those with the money to buy a high end printer as well as jump through the hoops and red tape to print an entire gun. That said, I see it more of a realistic benefit to those trying to get old guns up and working. Printing a sear or even a hard to find barrel could revitalize a current paperweight.
I would not discredit electronic ignition. Battery technology is improving not only in how well it holds a charge as well as how long it holds a charge and how durable it is. Computer tech WILL get small enough, light enough, cheap enough, and reliable enough for firearms in the future. We already "trust our lives" to computers everyday. We tootle around town fully trusting that our car's CPU won't ignore our command and just open the throttle up completely. We drive out into parts unknown without a paper atlas.Our phone knows the way. Some of us have little machines grafted into our bodies that bleed in our lifesaving medicines or physically keep our hearts pumping correctly.
This is what I see: An app based technology where a small sealed firing system receives near instantaneous signals from our communication device (calling it a phone is a little simplistic. These devices handle every major facet of our lives currently). The app is what controls the ignition. Touching a "trigger" on the gun is all that is needed. As wireless technology improves, so will reliability and response speeds. Universal wireless connection is already happening, and there is already a mathematically possible way to charge cell phones using wireless technology. Phones that are always charged and never go dead as long as they are under the umbrella of the wifi signal. For better or worse, biometric tech could personalize our weapons to the point where they won't operate without belonging to that person.
Does that mean the government could just turn off our guns? Probably. That means in 2069 you'll find my 88 year old self sitting in my high quality assisted living facility making crass remarks to the cute robotic nurse with an 1860 cap and ball stuffed under my high density foam smart-mattress
Is the weight difference between 20 rounds of 9mm brass case and 20 rounds of polycase ammo that big of deal for the average person?
In the military the biggest difference is transportation. A cargo plane can only carry so much weight. Ammo is heavy. A reduction there saves dollars.