So, we've gone through Codex: Tau Empire slot-by-slot, and taken a look at the worst units in each. Sometimes it's been a worst-by-default kind of situation, like the basically solid XV15/25, and sometimes the worst has just jumped straight off the page, like the Ethereals and the Vespid. Suggestions have been made about how to improve these specific units, but what about the codex as a whole? Codex: Tau Empire was published in 2005, over six years ago, and is old enough it still contains references to 4th edition rules. Since then the 5th edition codices have set a new standard for the game, and while some have been a bit lacklustre, like Tyranids and to a lesser extent Imperial Guard, the codices for Space Wolves, Blood Angels, Grey Knights, Dark Eldar and Necron have brought a variety of new options to players of those races, and gone some distance towards changing the conception of 'good enough' in Warhammer 40,000. Blood Angels and Grey Knights have made mobility and close combat even more decisive, while elevating survivability almost to the point of lunacy, while Space Wolves and Necrons have set the standard for mid-range, high-volume, highly-dangerous shooting. So where does that leave the Tau Empire as a whole?
Well, so far as I can see, it leaves it full of fragile units holding expensive guns they can't shoot very well, the most heavily armed and armoured of which can't fight their way out of a grot-pile and none of which have any psychic defences.
Now, one of these weaknesses is manageable, but two of them put a serious squeeze on a player, and the four of them together are just inexcusably poor codex design. It's hard to think of any other codex that has been specifically crafted to leave the best-possibly-designed army list struggling desperately to participate in one of the three phases of the game, or deal with basic issues like psykers. The railguns and plasma rifles and jump-shoot-jump capabilities may seem impressive at first, but at the end of the day you have an army that cannot fight, and therefore absolutely needs to shoot their enemies down as they charge across the board, but whose elites shoot no better than basic Imperial Guard troopers and who are as expensive as they are limited.
So, what should change for the next book? Well, obviously, a lot of things.
First thing's first, if Tau are going to live and die by their shooting, that shooting needs to be absolutely devastating. In terms of elite/anti-heavy infantry shooting, battlesuits should be BS4 standard across the board, and that includes XV15/25s and XV88s, and the jet pack units should be made truly Relentless. Plasma rifles should drop in points, given how essential they are to an army with no power weapons. An equivalent of the frag/krak missile launcher should be introduced; the Tau have very few weapons that can force Instant Death on a T4 model, and most of those are dedicated anti-vehicle weapons, of which Tau cadres never have enough. Some sort of anti-psyker munition should be available, something like the Necron's entropic strike that reduces or eliminates a psyker's ability to use their powers after successive hits. And as for infantry support/anti-light infantry shooting, if the Devilfish is meant to reinforce fire warrior squads it really needs a decent variety of load-outs; instead of a choice between various S5 AP5 18"-24" guns, where's the equivalent to the assault cannon, or the multi-melta, or the heavy flamer? Pulse carbines should be Assault 2, burst cannons should be AP4, and Smart Missile Systems should be assault weapons. Heavy gun drones should be introduced, not as a heavy support choice, but as upgrades to fire warrior, pathfinder or gun drone squads. The AFP should be made standard issue, and dropped in points; it's the only decent anti-horde weapon you could take multiples of without crippling the ability to respond to other threats. And all those are just off the top of my head!
But we can't spend all day talking about shooting, much as it would be a very Tau thing to do, so let's move on to fragility. Now, the Tau, and particularly the absolutely essential battlesuit squads, are fragile in three ways; low toughness, low numbers, low leadership. The first makes them very vulnerable to fairly common Instant Death-dealing weapons, particularly missile launchers and meltaguns, the second makes them vulnerable to constantly being forced to take break tests, and the third makes them vulnerable to routinely failing those tests they're constantly taking. There is a perfectly simple fix for those; increase battlesuits' T to 5, and increase all units' LD by one. Right there you've made them a significantly more hardy army, which is good, because losing a good chance at losing 186-point Fireknife squads to an 18 point Marine with a 15 point missile launcher is devastating. There's also an alternative, to lower the price and increase the size of units, so you could field four or five XV8s instead of just three, and get units of nine or ten fire warriors onto the field instead of units of six. I'm less enamoured of this, as getting XV8s into cover is often tricky as-is, and the armed forces of the small but dynamic Tau Empire are supposed to play as a moderately elite army, not an IG/Tyranid swarm.
With regards to pskyers, the lack of warp presence is, as they say, a design feature, not a bug. The Tau are supposed to be relatively cut off from the whispers of the Warp, so while the easiest recourse would be to drop some Nicassar or gue'vessa psykers into the book, it does somewhat dilute the flavour of the army. A better choice would be to have the Tau counter psykers technologically, trusting to their science over their enemies' superstitions. The aforementioned entropic strike-esque anti-psyker gun would be nice, for instance. As would a warp-dampening field generator that works like a psychic hood, something you could buy for battlesuits, or perhaps ethereals to finally give them some actual utility. And a piece of bespoke wargear that can go through psychic saves would make dealing with things like the Doom of Malantai or Eldar farseers significantly more reasonable. Psychic defences are perhaps the easiest problem to fix, because you can largely re-purpose other races' abilities, attach them to a piece of wargear, and call it a day.
If psychic defence is the easiest problem, though, close combat is the most thorny. After all, the Tau Empire's military high command disdains close combat, and the Tau themselves are famously awful at it, being shorter and weaker than the average human, with lousy depth perception to boot. But what is the point of auxiliaries if not to shore up weaknesses in the conventional Tau military? I mentioned, while looking at them, that giving the Vespid Rending and Hit and Run would make sense; they'd remain an at-best mediocre combat unit, but with some decent tricks and the chance to really participate. Well, there are other auxiliaries out there who can offer some help, too. The Kroot have a certain quality in their quantity, but making their guns Assault 2, or even just a basic one-shot assault weapon, would go a long way. What's the point of an assault unit that can't soften the enemy up with some shooting? And the gue'vessa, humans living within the borders of the Tau Empire, could lend a hand, as well. Load them up with pistols and close combat weapons, give them some decent armour (5+ minimum, 4+ ideal) and the option of special weapons and equipment, and turn them loose. And all of these choices, it should really go without saying, should be able to get access to assault grenades. Lastly, there are rumours of demiurg being included in the new codex, and if the rejuvenated space dwarf concept lends itself to anything, it's a tarpit unit. 3+/5++, T5 and W2 would go a long way towards giving the Tau something they can block dedicated close-combat units with, at least something that won't just evaporate as soon as the enemy looks at them sideways.
So, there are a host of suggestions for dealing with the various weaknesses Codex: Tau Empire has. Should all of them be implemented? Any of them? I haven't playtested them myself, so I can't speak very well to that. But if nothing else, these sorts of things are at least moves in the right direction, attempts to make the Tau Empire a little less vulnerable to being absolutely rolled over by certain, not-uncommon armies.