Here we will discuss UX/UI convergence (like Ubuntu Unity 8's one).
(this is a sub-topic of Plasma UX/UI)

First of all, we should decide which devices to focus:

1) Smartphone
2) Phablet
3) Tablet
4) Notebook
5) Desktop
6) TV / feature phone

IMO it's important to optimize Plasma for tablets and also for phablet. These two form factor are ignored by Android, its UI is the same on phone but resized. Instead we should take care of of them.
Moreover we should optimize Plasma for TV and feature phones, namely make Plasma usable through directional buttons.
We should do this sharing 100% of the code between Plasma versions (so there will no longer be "versions", but only "Plasma").

I don't know why nobody has answered this post yet but I would definetely focus on Desktop, followed by SmartPhones. A mockup with 4 main devices (TV, Laptop, Tablet, Smartphone) showcasing Plasma 5 user interface would be great in order to have an idea of the looks.

Created this wiki page, https://community.kde.org/Plasma/Convergence_Overview, with an overview of the different device types. I think that there are 3 device types that are very different from each other, Desktop, Smartphone and TV. I think that you should keep all 3 device types in mind in order to ensure that Plasma can transition between them.

However, if you're asking for a single device type to design for, I'd have to say smartphone. Plasma has implicitly always been focused on the desktop device type, since that's what it was practically always running on in the past. So Plasma should already be optimized for desktop devices. Next it should focus on designing an optimal user experience on smartphones, without ruining the experience on desktop devices of course. This should result in a design that allows for a good user experience on both a desktop and a smartphone. Of course, it would be best to also include tv's as device type in this design, so you end up back with designing for the 3 device types I mentioned earlier.