For major advancements, integrally suppressed pistols. The Maxim 9 is a precursor, but an ugly and impractical one. It's not hard to see that if suppressors were deregulated even in a limited market the demand would extend far beyond screw-on cans. It would also affect ammo. Possibly we'd just see renewed popularity for 147 gr. 9mm vs 124gr, but there seems to be plenty of willingness for makers to take a crack at introducing new cartridges.
In another area, electronic ignition. This one would qualify as a major advancement but maybe not as the "next" one. I think it's easier to achieve than caseless ammo. Chemical primers would be replaced by battery-powered ignitors. With electronic control of ignition there would be revolutionary benefits. The "action" of a gun would be decoupled from the trigger. It would be "fly-by-wire" or "fire-by-wire" so to speak. The trigger would become a simple switch that could have any mechanical properties one desires. There would be no hammer, no striker and no springs needed to ignite a chemical primer, so cocking and energy stored in springs would be done away with. The only mechanical operations would be loading, recoil absorption and ejecting. Firing would be much faster -- meaning less time between the trigger pull and ignition. We're not always aware of the long delay while the hammer or striker falls and the chemical reaction in the primer begins until the powder is ignited, but it's there and it affects us. With electronic ignition, we'd also inevitably see computer control of the ignition -- meaning authentication, authorization, and accounting could be applied to the ignition procedure (for better or worse).