I have been thinking a lot about other platforms. Each codebase is going to end up with its own section in Redmine, but I don't intend to create those sections until development begins (because there are a lot of possible options, and I don't know which path I am going to go down yet).
I have written a bit about this topic in other places, like the roadmap, but I will add a few thoughts here as well.
The next platform I intend to develop for will be the desktop. Originally, I was planning to wait until after the 4.x series was complete before starting development for desktop. But I am now thinking that I might move that up and start desktop development after the completion of the 3.x series.
My current thought is to develop the desktop codebase using the QT framework (this is the same framework used by KDE). My primary target would be for Linux, but I would also build and test extensively for Windows. QT can also be compiled for macOS, and I would release on that platform as well as long as it didn't require too many customizations.
Mobile Linux platforms that ship the QT libraries could also run the desktop version, but it is unlikely that the UI would work well on a phone sized screen. Rather, after getting the desktop version into decent shape, I would likely need to build a separate version that works well for mobile Linux. In fact, because each mobile platform is so unique, I might have to build a separate app for each platform that takes off. Hopefully there aren't too many of them.