I think what you should do is set yourself a goal. Make an army list including what you have at the moment, and some other units you think you might want. Set a target, Probably 750 points glancing at what you have, and just work towards it. But I'd go with what Doktor Rummy said, another Crisis Suits and more Fire Warriors. These basically form the 'core' of any Tau army, so they're good to have. But I mean if you want some variation or something different, maybe a Piranha then.