There have been quite a few writing tools that are effectively just GPT wrappers with pre-defined prompts. "rephrase this more formally". Personally I find them to modify too much or are difficult to use effectively. Asking a for a few different rephrasings and then merging it myself ends up being my workflow.

But ever since learning about word2vec, I've been thinking that there must be a better way. "Push" a section in with the formal vector a bit. Add a pinch of "brief", dial up the "humour" vector. I think it could create a very controllable and efficient writing tool.