I just got news from the Threema-Channel… They are going Open Source!
“Within the next months, the Threema apps will become fully open source, supporting reproducible builds. This is to say that anyone will be able to independently review Threema’s security and verify that the published source code corresponds to the downloaded app.”
I’m no developer, but here are many… maybe there will be a way in the near future to build a native secure Threema messenger for the Sailfish-platform. That would be so awsome!