Any plans to allow inserting images (i.e. a signature)? Filling out PDFs is a huge pain for me and I'd love to replace it with this, but I need to insert handwritten signatures, not just text.
A signature pad could work too; I think HelloSign has something like that. But it's not a great solution if you don't have a tablet, since it's really hard to draw a proper signature with a mouse. Honestly, even just adding a "handwriting font" might be good enough for most circumstances. Most of the document signing services provide this, and it could be implemented very quickly with minimal changes to your API.
If I were you, I would probably start with a handwriting font, and add hand-drawn signature support afterward. I would also look into what the law says about this sort of thing. I know that services like HelloSign require that you authorize them to use your signature in a legally-binding manner. But I don't know if that regulation would apply to your service, since your service is a lot more bare-bones. For example, you don't store the PDFs indefinitely, and you don't coordinate any sort of multi-party signature operation; that all happens out-of-band. So you might be in the clear.