There was talk of 'voice engine' and the trademarks and details for that recently, I wouldn't be surprised to see that, mixed with the Gpt4-lite model which seems to be floating around (possibly GPT2-chatbot), maybe the lighter model is needed to allow low latency and also to lower compute costs.

I'm hoping that free chatgpt will move on from ChatGPT 3.5 in the process as well, as it seems there are two versions of the new model, I assume the free ChatGPT gets the smaller model and the paid one a better model which seems to be doing really well with more complex tasks, perhaps we shall also see agents involved.

To me, it's got to be fairly big news for them to have an event, it's not often they do, so it is a significant release which Sama has tweeted is like 'magic' but not search (as previously rumoured) or GPT5.