The remaining options are perhaps "weaker" but interesting flavour choices that may well have a place in a themed army.
1) Commander R'Myr
The second named character in the book, R'Myr is perhaps less "unique" (being an ordinary Crisis Suit as opposed to a character variant of the XV9) but nevertheless has a number of special items and rules. His loadout is perhaps poorly optimised compared to what is possible in the regular Tau Codex - the unique Double-Barrelled Plasma Rifle loses out compared to two plasma rifles (the same firepower at 24" and 2 additional shots at 12") and he cannot take systems such as the Stimulant Injector to supplement his 3+ armour save. The remainder of his items - an upgraded shield generator offering a 3+ invulnerable save in close combat, a target lock and drone controller and a flechette discharger (acting as a questionably useful variant on the Repulsor Impact Field) all support an aggressive use in melee, but are combined with a middlingly powerful mid-range weapon with no incentive to close in.
However, his main virtue is in fact his low point cost; as roughly the equivalent of a Commander with Target Lock, 2 Plasma Rifles and a Shield Generator (plus the bonus Repulsor Field and Drone Controller) he is significantly fewer points to field. Thus he offers a difficult choice for a Tau player; he is a cheap way of getting a moderately powerful suite of abilities in a HQ slot, and with a Target Lock can sit in a more optimised short-range bodyguard or Crisis squad and snipe characters.
2) Variant Crisis Suits
The two variant Crisis Suits (the Iridium Armour now rolled into the main codex) remain largely unchanged; the opportunity to take a Smart Missile on a Crisis Suit gives a significant anti-infantry firepower boost over the Burst Cannon, while a networked markerlight and Target Lock is useful for a more support-oriented Commander and synergises well with the other support items in the codex.
3) Drone Turrets
Drone Turrets were exceptional in the mess of FAQs that comprised Tau Imperial Armour support; as non-vehicle Troops choices with high toughness and saves plus the capacity for cheap Disruption Pods, they were a useful distraction unit, hard to kill and benefiting from the rate-of-fire increase for Burst Cannons.
While they remain Troops choices, they have been returned to being vehicles, and increased in cost. Ultimately their poor BS combined with lack of resilience (armour comparable with most light vehicles and few hull points), and the switch back to paying regular costs for vehicle upgrades (thus Disruption Pods triple the previous cost) mean the previous useful cheap distraction is now more points than a Hammerhead for a significantly less viable unit. Were they Fortifications, or if they had received some increase in firepower, there could be a use for them - yet with the current cost they are simply too expensive to field and too inaccurate to really make their points back.
4) Sensor Towers
Sensor Towers, unlike Drone Turrets, remain as a kind of Artillery-type unit; with extensive special deployment rules (essentially a 6" unit coherency and the capacity to fire mounted weapons without an operator nearby) they are among the first faction-specific Fortification choices in Warhammer 40k. Coming in units of up to 4, they provide a way of bringing many twin-linked Markerlights to the battlefield without using up Fast Attack slots. A Tau army light on Pathfinders or Marker Drones would get much use from Sensor Towers, and their high toughness makes them much more resilient than most alternative ways of deploying markerlights.
On top of this, the Sensor Tower allows a unit within 2" to re-roll 1's to hit and gain Night Fighting each shooting phase; a small bonus, but very useful given the prevalence of Gets Hot weapons in the Tau Codex.
5) Heavy Gun Drones
The Heavy Gun Drone is apparently obsoleted in the Tau army by the presence of the Remora Drone Fighter. For just over a quarter of the points cost it offers half the shots, no Seeker Missiles, and, none of the resilience of a Shrouded Flyer. While it may upgrade to take a Markerlight (while losing Twin-Linked on its burst cannon), its low BS mean that it is simply too inaccurate to capitalise on either weapon. Furthermore, as a Heavy Support choice it is overshadowed by far more efficient anti-infantry choices such as Broadsides and Hammerheads.
Perhaps its only virtue would be as an escort for a model with a Drone Controller, although it is a pricy investment and there is no clear ruling as to whether the combination even works.
6) Piranha Variants
The Piranha TX-42 is a definite upgrade to the regular Piranha and has benefited especially from the changes to rail rifles. Its main advantages for a points increase of over 50% are an extra point of side armour (less useful than it immediately seems since the speed of a Piranha makes it a one-shot suicide unit more often than not) and a twin-linked main weapon in place of the gun drones.
Perhaps the best use for the TX-42 is with twin-linked Rail Rifles or Plasma Rifles; while the cost quickly increases per model, the amount of AP2/1 firepower that the unit can put out at a decent range makes it able to kill significantly more than its points cost, while being somewhat more resilient than a Pathfinder squad. As with many new Tau units, though, a high points cost limits the capacity to deploy them; three TX-42 with Rail Rifles is the equivalent in points to almost two full 10-man Pathfinder squads with three Rail Rifles each. In a thematic list, heavily built around light vehicles, they would find a niche, but in a more balanced list it is expensive for only a questionable advantage.
7) Kroot Units
The added Kroot choices in IA3 are both thematic (allowing for an almost entirely-Kroot army) and diverse; adding cheap monstrous creatures to the Tau army gives a little melee punch and disposable bodies to deflect attention away from more powerful units. In this way they serve a similar role to the Riptide; models intended to soak fire, deal some damage, and escort the remainder of the army.
Knarloc Riders are light cavalry, able to take sniper rounds like regular Kroot but also somewhat more powerful in melee with S5 and 3 attacks each. In units of up to 9 they are less effective as a shooting threat but probably the strongest fast melee option in the entire army list - Stealth while in forests and the ability to outflank, however, do not really do enough to make up for their poor armour save and only average Toughness, easily victim to Instant Death.
Goaded Great Knarlocs are a pure melee Monstrous Creature, capable of taking an average armour save at the cost of one attack. While their stats are poor compared to a Tyranid or Daemon creature, they are resilient enough and hard-hitting enough to serve as an adequate distraction unit and can soak wounds on a unit of up to eight attached handlers. However, to make up for the low costs of these units, there is a chance that in melee the Great Knarloc will attack its own unit. On the whole, though, it is a fun thematic unit which also fills a niche (fast, heavy-hitting melee blob) that the Tau lack.
Mounted Great Knarlocs are even more powerful; coming in units of up to three Monstrous Creatures, each armed with a bolt-thrower that can in turn be upgraded to shoot blasts (and with one in three taking the Autocannon-equivalent Kroot Gun). The result is a unit of models with enough wounds to soak enemy fire, the capacity to move rapidly through cover and hide in forests, and powerful ranged attacks to cover their advance.