I would love to have a native Signal/Textsecure messenger, I think it's cleaner than telegram or Threema and of course Whatsapp, since it's fully open and documented.
And yes, I would, like nthn, prefer to have it integrated into the messages application via telepathy (even if the messages framework on Sailfish OS would maybe have to be beefed up for pictures, media etc.)

Edit: for those who speak Go and want to port the Ubuntu thing, there seems to be a Go runtime for Jolla, here is a nice description:
https://together.jolla.com/question/...t=votes&page=1