Drone / Sentry towers could easily go into a variant list that focuses more on gun drone units (Which would also have units like the Drone Harbinger and Heavy Gun Drones).

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Currently doing a plastic scenery kickstarter

Always been a big fan of the drone heavy list. It would need those drone fighters available from FW.

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author of Syncing Forward and other stories...

It's a dog-eat-dog world, and I've got my Milkbone underwear on.

This has already been answered very well by others but just to reiterate - these things are mostly not in the current list to keep it focused (easier to balance) and so there are interesting things to add to future variant lists.

A selection of possible stuff for future lists

Stingwings
Harbringer Drone Transports
Great Knarlock
Heavy Gun Drones
Variant Battlesuit configurations
Variant Hammerheads
Sentry Turrets and Sensor Towers
Scorpionfish
Drone Fighter aircraft
Human and other Alien Mercs

and I am sure many more.

This was I think brought up by GlynG, I believe there is background at least from forgeworlds IA that does indicate they can be transported by Mantas but not Orcas.

I have to admit I personally think the idea of Tau letting these sorts of things onboard any of their transport craft as a bit silly (and certainly I dont envy any Tau technician 3rd class who has to clean up the mess ).

So just decided to leave the list as it was on this, ie no transporting Krootox. Potentially also they add significantly to the assault capabilites of a Kroot formation because they bring 2 x 4+ attacks but more importantly add armour saves which can have a big effect in an assault.

This might be a bit left field but has anyone thought about making them a free formation like the Avatar or free add on units? They don't have to have super stats and you can make them move the same speed as the formation they join with a special rule? (I know it's another one)

Sorry Dobbsy, just to check is this a suggestion on Drones? Just in the context of the posts above I cant be sure - it seems as though you are suggesting free Krootox for the Tau army which would indeed be a bit left field

Edited - just realised I think you are talking about the Drone/Sentry Towers. If they were introduced to a variant list I think that would be a reasonable way to add them, something to do with objectives or a single free set etc

Sorry, yes drones was the formation/add-on. As drones are ubiquitous a couple of units here or there might be the better way to intro it.