I agree with Cammerz, combined arms are meant to be the Tau's thing. That said, Seekers and Markerlights could do with some improvements to my mind. Maybe a short range pistol version of the markerlight to make it easier to get emergency hits off, and seeker variants, like an anti infantry missile, more access to destroyer missiles outside of superheavies, etc. Also Stratagems, like something that allows another unit to fire out of turn when an enemy gets within 12" of a unit you designate as the Lure during deployment (a true Kauyon rule, rather than the boring re-roll aura we have at the moment).
To be honest (and getting a little off topic), stratagems are something I feel could use a lot of work with the Tau. Of all the factions I feel the Tau deserve a bunch of powerful stratagems that require tricky circumstances to pull off. Like "use when X unit fires on an enemy with at least Y markerlight tokens and has friendly Z unit within 12 inches". I like the idea of the Tau really benefitting from having a plan and successfully executing it, it fits their theme and would help differentiate them from other factions