Having it work more reliably and with increased functionality, for a start. It kind of feels like Apple hasn't made any meaningful improvements to Siri since it launched - it's just kind of languished when compared to other areas of the iPhone experience (faster chips, better battery, better camera, more customization options, etc.).
It just seems that Siri is way behind all of the other stuff that Apple has been doing, and they could build something that's head-and-shoulders above the competition. That's the missed opportunity - I'm thinking of Siri from a utility standpoint, not in terms of how much money it generates for Apple.
BTW, I'm not answering a question with a question. I am replying to the sentence you wrote AFTER the question.