Here are really quick suggestions for Jolla, some small additions that, thanks to their overpowered SDK, might take less than 10 minutes to implement, but are still widely asked.
Sometimes, when you received a text message, and read it in the messages application, hub or whatever, you might want to call the person who sent it quickly. This feature, that many people find important, is often forgotten in the design process. Not providing it it is a bit strange since calls and SMS were related since the beginning and still appear together in all contacts applications.
Adding a button in the message interface might be a bit messy, adding a pully menu with that option would be great though.
Writing SMS on touch screens are sometimes a pain, since you might accidentally hit the send button, or write a typo and hit send before realizing it, leading to those autocomplete-fail sceenshots that you can find all over the Internet. Symbian used to provide a delay in which you could cancel an SMS, it might be nice to have the same options inside Sailfish.
Swipe to close, swipe to switch ?
I know that Sailfish UI is still in progress, and that UI might change in the future, but I want to say that Jolla must not forget some of Harmattan strength, like swipe to close. Although most applications deserves to be kept in the background, some of them, like the GPS, might need to be closed at some point The two steps operation, minimizing and closing is a bit slow compared to the quick swipe-down-to-close in Harmattan, that was both intuitive and easy.
Another nice swipe that BB brought, and that I felt a bit overkilling is the swipe to navigate between apps. Actually, after using a Playbook, I felt that it was actually rather nice and might please power-users. MS adopted it as well in Modern UI in their 8th iteration of their OS.
These small additions can make people life easier. And actually, the UI is meant to do that at first, to enable users to interact with their devices in an efficient way. I still applause the different innovations provided by Sailfish UI, but also think that adding these features will make the UI even more efficient and pleasant to use.