I've always liked the look and feel of Tau, and so, collected a fair number of models. That said, while the idea of a gunline appeals to me, it's something that doesn't particularly fit my play style - I prefer to be mobile and make things happen on the board. With this in mind, I wanted to ask myself, is there a way I can build a mobile, slippery Tau force without losing its effectiveness.
The first thing that will have to go from the "traditional" Tau list build is Broadsides. Skyrays too, though I think they're a little less popular at this level, and a bit more mobile. The issue with both is that they need to be stationary to be most effective. For the broadsides, plenty of markerlights can offset them moving and snapshooting, but that is a bit of an unnecessary hurdle.
One strong, mobile platform the Tau have is, of course, the Riptide. Paired with a buff commander he is mobile, durable, and more effective.
As far as Troops, Kroot are obviously mobile with their infiltrate ability, but nothing about the Fire Warriors outside of the need to keep them out of combat necessitates that they stay in place. Additionally, moving your firewarriors can help keep your opponent off balance since usually those units are static.
For markerlight support, sticking with the mobile theme, I don't want Pathfinders. Having a heavy weapon on infantry would pin them in place, and as a key support unit, provide an easily exploited weakness to my opponent. In this build, if I went with pathfinders, it would be small squads with upgraded weaponry outflanking.
The other obvious choice for markerlights then becomes Marker Drones. However, they will require a babysitter of their own. A commander with Drone Controller is the obvious choice, but extremely pricey. An alternative would be a single Crisis Shas'Vre with Drone Controller and Command and Control Node. Let's do some Math-hammer, comparing the output of the same number of points of babysitter and drones a few scenarios, with a cap of 188 points (min drone squad and Commander w/C&CN and drone controller).
Commander w/drone controller, C&CN, and 6 total marker drones: 188 points, 5.833 hits per round
Commander w/drone controller and 7 total marker drones: 187 points, 5.833 hits per round
Shas'Vre w/drone controller, C&CN, and 9 total marker drones: 177 points, 6.75 hits per round
Shas'Vre w/drone controller and 10 total marker drones: 176 points, 5 hits per round
13 Marker drones: 182 points, 4.333 hits per round
The obvious winner is the Shas'Vre with drone controller and the Command & Control node. If you divide the points by how many hits you get each round, the Shas'Vre setup nets you a markerlight hit for every 26 points spent on the unit (compared to 32 from the commander and 42 with drones alone). It also has more bodies which can maintain effective output even after taking a few casualties. And with JSJ, this unit should be able to be mobile and stay out of trouble.
Finally, I'll round out the list with some stealth suits, which can put out impressive volumes of S5 firepower and are tricky to root out with their 4+ cover save in the open (which they shouldn't be most times).
That leaves me the following list:
Commander: Multi-spectrum sensor, Puretide Engram Neurochip, Vectored Retro-thrusters, flamer
Riptide: Ion Accelerator, Early Warning Override
Crisis Team: Shas'Vre, 2 marker drones, Burst Cannon, Command & Control Node, Drone Controller
5 Stealth Suits: 1 w/Positional Relay
8 Fire Warriors: Pulse Rifles
8 Fire Warriors: Pulse Carbines, Devilfish w/Disruption Pod and Sensor spines
5 Marker Drones
This list is designed to cause an enemy problems in their own side of the field. One or both Kroot squads stay in reserves while the Stealth Suits infiltrate forward. Turn 1 sees the Riptide and commander jump forward while the Devilfish goes all out up a flank. Shooting sees the Riptide and Stealth Suits focus on one or two high priority targets with the Stealth suits usually using some or all of the Marker lights to maximize their fire. Finally, the suits all jump either forward or further into/behind cover to avoid reprisals.
Turn two should see one or both Kroot squads coming on and being able to come on the opponent's board edge if I place my Stealth suits well enough ahead. The Devilfish will let out its passengers if I want their firepower, or continue to buzz along quietly to jump on an objective in later turns.
The strength of this force is its mobility, and by being aggressive early on, most of the army will have the space to flow away from enemy strongpoints and toward weaknesses, staying more or less just out of reach of the hardest blows the opponent wants to throw. This force also is a counter to many expectations an opponent would have in facing Tau, and can capitalize on unbalancing the foe just a little on those grounds.