I was idly thinking about Tau weapons systems with the current codex, and it occurred to me; why don't the XVs carry rail rifles instead of plasma rifles, and a similar question; why missile pods instead of Ion rifles?
It actually started out with "why don't they carry them at all", then I considered the weapons systems largely overlap in function and purpose.
So here is the reasoning
Rail vs Plasma
Rules wise the two are very similar in function and points; the Rail rifle has a slight edge in AP and range. The AP difference only really helps for pens against vehicles, and the range difference is not a huge advantage given the size of the normal battlefield and mobility of forces. It also fits better with the Missile pod or Ion rifle for range bands. Gamewise I don't think a change would be a major game breaker (though since this is just a thought experiment, I've not tested it) and it would simplify the codex by eliminating one weapon (that is unique to XV suits)
Fluff wise, the plasma rifle does not really fit with the Tau weapons systems; unlike the imperium there is no family of plasma weapons, and the rail rifle fills the same niche. It is a stand alone weapon option , so replacing it would tidy things up, and help unify the look and feel of the Tau forces. The Tau now really have 3 Iconic weapons groups; Rail, Ion and Missile. Switching to a railrifle would simplify maintenance and resuppy, and would give the iconic XV series the iconic Tau weapon; railguns. Unless the plasma rifle is significantly cheaper and/or more reliable (hmm...High energy plasma physics...) theres no fluff reason to retain it that I see. The only real advantage to the plasma is no real ammunition requirement, but railrifle ammunition is not exactly bulky, so not a major advantage.
From a modeling perspective, it would seem like an ideal opportunity for GW to repackage XV-8s with the rail rifle replacing the plasma which should see a small boost in sales as people that already have suits get new ones for the rail rifles, or they could sell separate weapons sprues (at a noticible mark up) for conversions. It would be a relatively minor modification to the moulds (change the front of the plasma to a rail rifle barrel, done.) so should not be too expensive or difficult. As mentioned in the fluff comments, it would help unify the look of the tau forces.
A similar argument could be made for replacing Missile pods with Ion rifles, though the argument is not as strong; The ion rifle would fit the aesthetic of XV-8s better, and would be a more logical choice in many ways; missile ammo is bulky, making it a poor choice for sustained ops. The missile pod is the only real long range weapon for what is generally a special weapon rather than heavy weapon platform.
The weapon profiles are similar; the ion rifle loses a little range and a shot over 1/2 range, but gains the potential for blast and/or better antitank penetration, with a risk of overheating. The biggest advantage to the missile pod is in an air defense role, or just long range weight of fire. Switching XV-8s to ion rifles would reserve missiles for heavier platforms (XV-88, vehicles) again streamlining and unifying the Tau aesthetic.
I'm thinking to replace most of my plasmas with Rail rifles (if I can find enough spare parts) and just referring to them as Railrifle-LW's (Light weight) just as a style thing; "to adapt them to the battlesuit, the power to the magnets had to be slightly reduced to prevent interference with the suits power systems. As a result the penetration and range suffered a small reduction, but the difference was deemed within acceptable parameters (ie plasma rifle stats)"
I originally made a suit rail rifle using two flamer barrels end to end (surrounding it with shielding and cooling systems), but will now try to hunt down surplus rail rifles...