Tau Empire tech and ideas are really cool, but the allied species/auxiliary species are always cooler than the actual blue-skin Tau themselves. The caste system and their biology and all that is boring as fuck, what's cool is an alien confederacy made up of a bunch of benchwarmer species that just want to not live in a fucking hellscape. Leaning into that would be the best thing GW could do for Tau sales, just decimate nearly all of the actual Tau in some fucked up incident so they're a tiny minority in their own empire. Make the greater good and the sleek tech the only consistent part of them, with different flesh beneath it every time you see them.