Much water gun tech is a matter of cost. For most, air pressure and CPS are the best methods within reach of the wallet. Generally, the most high tech water gun ever designed is the CAP
which runs on regulated HPA. Motors and hydraulics would likely add too much weight to a water gun and remember that as water output increases, the power needed to launch it farther increases exponentially. In other words, you get diminishing returns for every bit of power you add to a water gun; diminishing returns to both range and velocity. (Velocity and range can conflict with each other, and the best balance between them is debatable.)
If you don't want to deal with pumping time between shots, the best way to do it is to pre-pressurize everything, where all water you carry is pressurized beforehand. The most common example of this is the SC Power Pak, which consists of a CPS backpack and a tube leading to a small gun that's essentially nothing but a trigger/valve and nozzle.
Larami used to be at the forefront of water gun technology development. They made huge innovations but after they merged with Hasbro, the quality of Super Soaker products took a massive dive and very little innovation has taken place from them thereafter. Buzz Bee Toys, started by Larami employees after the merge, created their own line of blasters, the Water Warriors. BBT has created unique design concepts from this line over the years, such as a pre-pressurizing air system, the Hydro Power pressure chamber, the bladder embedded in the reservoir, and the spring-powered pressure chamber. None of these deliver the next generation of water blasting, but they are innovations nonetheless.
At the forefront are modders and homemade builders. Ben at SSC, who created the CAP, also created the SuperCannon II
. Many others have created ergonomical homemade designs, water balloon launchers, etc. and the DIY community is well and alive with new ideas amongst those who are working at homemades.