@warzan About that Grav Stug. I really like your ideas for that and would love to give you my opinion on them. First of all. White tanks looks awesome. Over here at the German Panzer Museum they have a white UN Keiler (that is in actual service as I understand it and is just on display as long as UNO doesn’t need it to rip up any mine fields) and it is one of the most awesome looking tanks I have ever seen.

That being said, white and fast would imply White Scars. And White Scars would have been my second choice for my own army after Howling Griffons. The Scars would absolutely use grav vidicators and tank riders if they could. And I would still love to do a White Scar, or one of their successors, army an go really heavy on the Mongolian imagery with them, but I will probably never that. I would make them look really tribal and nomadic and add some tibet buddhistic and animistic feel into it as well. A lot of the respective imagery has that some dark feel that 40k sometimes invokes, where you are like “So those are the good guys? Do I really want to meet the bad ones?”

But I have another idea, that would be even more in the gaps. A force from a lost people (humans that have at least initially never encountered the Imperium Of Man after old night, who are now fighting alongside the Tau as irregulars, being supplied and supported by them, but also relying on their own technology, not being beholden to the cult mechanicus. The Tau might not have bothered to conquer them due to their relative military strength, but might be using them as a buffer or a war proxy. You could make them up from parts of imperial guard, other 28mm soldiers and old school marines and some Tau weaponry, making them baseline humans wearing astartes-grade power armor of their own manufacture. That is something that comes up a few times in the 30k lore, when the legions are eerily reminded of their own equipment by the looks of some of the non compliant troops they are up against. Rulewise one would of course have to make up a mix of Guard, Marines and Tau to play them, but it might make for a fun force.
The Imperium likewise has more pressing military needs, than to crush them, but will eventually get around to it. Until then it knows they are out there and rogue traders do maintain unsteady contact with them, maybe even getting them to fight alongside imperial forces from time to time. They would be accepting weapons shipments from both sides (Imperium and Tau) and be acting like one of the block free states of the cold war.