Just to add a bit more flavour, I'd interpret these as;

Frontier world - The new colonises and/or worlds taken from the Imperials during the 3rd sphere expansion campaigns. They've not got the population to support large armies, nor the stockpiles of traditional weaponry the major Septs have. However they are the most recently colonised and thus tend towards having the latest tech.

Lots of drones, Remoras, Hazard suits, fusion equipped XV25 stealth teams backed up with the Gue'vesa and a few Firewarriors or Pathfinders (maybe with the odd Stealth Optimised Devilfish from Apoc). Basically, this is also where most of the new stuff that's come out for the Tau in Apocalypse in FW. I'd be tempted to throw in a few odd ball things here like the defence towers (since they're a type of drone) and maybe even have different space craft with orbital bombardment (the Darkstar and Icefire warheads used to EMP blast vehicles or destroy organic life so technology/structures can be preserved and salvaged). So a very Kauyon focused army, as Farsight fills the extreme Mont'ka niche. Perhaps even a little 'gimmicky' so a lot of combinations and interactions between smaller scale ML-type abilities. An army for the people who found the main appeal of the Tau was the synergy between their units around the ML and Co-ordinating fire mechanics more than the mobility and firepower.

Armoured interdiction force - This is kind of the next generation of the 3rd sphere expansion force from the major septs, with the various 'experimental' designs that got dropped during the development of the previous list. People seem to have a pretty good handle on what's going here, it's the army for people who have all these models and found some elements of the older style of lists a little more appealing than the current one. I'd almost be tempted to have a lot of integrated markerlights (leader of the armoured cadres having them, maybe even satelites-type flyers) and a focus on units with positional relays - which then guide in the larger guided missiles of the Stingrays and Scorpion fish to get it a lot closer to the original ML mechanics if that really is the focus behind this list.

Farsight Enclave - Farsight is famous for his dedication and perfection of the Mont'ka, and his experience fighting the Orks who aren't willing to sit back and let you shoot at them. Since battlesuits are already a core cadre in the standard list and Farsight is (in 40K) heavily restricted in his access to ML equipped troops, I'd be tempted to use this as an opportunity to explore the Tau firefight army. In 40K, Enclave Firewarriors have free bonding knives and Ork fighters (reroll to hit Orks in cc) so upping their FF value and their ability to handle blaster markers could be a way to create the backbone of a very different style. These guys can then be supported by Battlesuits with different weapon loadouts (more flamers, burst cannons and fusion blasters) who have a more FF or AT focus than the current unit. Maybe a few other new units that have come from them trying to salvage as much of their old technology as possible, barely flying Orcas becoming weapon platforms recovered from damaged Hammerheads.

Auxiliary - I'd hold off a bit on this one perhaps, since there's a lot of pretty strong indications that the Tau will be getting at least one new alien race (probably Demiurg) which would potentially change the way the list plays quite heavily if they're a slow and heavily armoured unit or race. I think the biggest question is what purpose does this list serve, other than because of units that exist in the background which aren't represented in Epic? There's now quite a lot of units that are only vaguely mentioned in the background which have the potential to fill out an entire army alongside the Kroot if we wanted - but it'd be pretty much designing an army from scratch (depending on how much Tau we wanted to keep in there). Just then a case of finding a niche that needs filling or some interesting mechanic.