Specialized sidearms and/or special-use soakers can ruin iSoaker's question lol.
Actually, no, it wouldn't; it'd just add another layer of complexity to the question, but one that'd be great to explore as well (as I'll do below).

While range is undoubtedly important for a lot of water blasters, I think DX summarized my own thoughts best so far with the statement:
An underlying factor is that a good water gun should be practical for its intended uses. These vary too greatly to really define, but basically, the water gun should empower the user to their maximum potential in a given setting.
I would have used "The" as opposed to "An" for the opening word, but that's me.

While there's always a general demand for a decent, all-purpose water blaster, there's a lot of room for more specialized blasters to compliment different game types or water warfare environments.
A good, all-purpose soaker to me would be a decent size, somewhere between an XP150 to a CPS1000/1200/Orca. It should have a decent capacity (2-3L) and a decent effective range (preferrable 35-40'). Output isn't crucial, per se, but in order to be able to achieve the range sought, it'd probably need a 3-5x stream for one of its settings. A multiple nozzle selector would be nice on such a soaker, but not an absolute necessity. If anything, being able to swap between a stream to a max-output burst is nice, but not needed for an all-purpose soaker. As for pump volumes, as larger soakers should optimally be targeting an older audience, a larger pump volume should be possible, but not to the point that pumping gets too energy requiring. The best air and water pump volumes I've used are on the Super Soaker XP75 and Super Soaker XP150, respectively, pushing ~75mL per full stroke. The worst pump was when trying to pump up the first generation Water Warriors Aqua Master Krypton after pre-charging it; that was particularly difficult. For a separate PC blaster, it should preferrably take only a few pumps to yield a good stream while completely pumping up the PC would yield optimum performance. Pressurized reservoir systems should be used only for pistol-class soakers, thus should also not need too many pumps to achieve adequate pressurization.
As for specializing, it'd be sweet to see soakers that are better suited for specific tasks. A Flash Flood that has a slightly more contained nozzle, but no standard stream nozzle, would become the "shotgun" of the water blaster world having shorter range (aiming at between 20-25' effective range), but high output and area effect. A longer-firing water blaster would be nice to have as well with the output and lamination needed to produce a stream capable of reaching 50'-60' effectively, but a shorter total shot time so as not to compete with a general purpose blaster. I could also imagine at least two types of pistol-class water blasters: one would be like an XP220 in size, but with a slightly larger nozzle; another would be like the Triple Shot set solely to its largest setting as a true last-stand hold-out blaster.
There is, of course, always room for larger cannons and tanker-type blasters. These would be balanced in a water war game in that they'd be larger, thus heavier to wield, thus a little slower to move with. As well, by being larger, they would also likely require more pumping between shots, thus rate of firing would be reduced while output would be more.
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Feel, balance, and styling still play a key role on what makes a good blaster. While some are more than happy to personalize their soakers, others like keeping their soakers stock and want them to look slick as they are. In terms of balance, this also encompasses general ease of use. A good water blaster should feel sturdy and comfortable to hold when empty and when full. Straps should be a given for any soaker longer than a Super Soaker XP110, IMO, while some sort of holster should be available for smaller armaments. While some may prefer closer to realistic gun styling, I prefer optimal water blaster designs. That is, designs that truly consider making the most of the space and functions desired. The clean lines and form of the Super Soaker CPS1000 make use of pretty much all of its parts while offering superb feel and balance. While I do like the XP 150, when loaded, it is more top heavy and less inherently balanced. I do like the clean lines and styling of the Max-I Defender, but its PC shape is sub-optimal; had it used a more bottom-tapered PC, it'd score more points.
These are just some of my thoughts, but far from everything. You guys have brought up a lot of good points and ideas. If you got more, keep the thoughts flowing! You never know who might be listening and considering...
I still believe that despite the present lull in terms of stock soaker performance, there's remains a chance for something better.
