I had an idea for a book featuring the tau and whatever Imperial counter element dan abnett or sandy mitchell can competently throw together... It goes like this...

The Tau are on the verge of their 3rd and 4th sphere expansion and have been toying with the idea of controlling the Tyranids as bio-weapons... They have a secret base on a remote moon to conduct their experiments, the IG or the Spazmareenz can come in, mess stuff up, kill the bugs, squich the Tau, hug their emperor, and save the day.

This is something I'd read which I've never seen in a 40k novel. Perish the day where we change stuff up a bit...