Plasma lead developer Seigo explains that, "This is a great opportunity to get changes in that polish things up as they will be available for a long while. Often between releases whole components are revamped and sometimes this results in some polish being lost temporarily. With a long lifespan, these improvements will be allowed to naturally accumulate to the benefit of those using it."
This statement does not preclude changes visible only to developers -- and KDE is, in fact, gearing up for Wayland support, just as GNOME is. Still, if this intention is kept, then visible innovation will not be a defining feature of KDE for the next couple of years. Any major changes will probably be on the periphery, such as an app store, rather than on the desktop itself.
The largest innovation in KDE will probably be the release of Vivaldi, a tablet computer loaded solely with completely free software. If successful, this tablet may highlight the Plasma Active interface, as well as giving KDE's concept of task-oriented Activities the publicity it needs to become widely used. However, after over a year of constant delays, the question is stating to arise of whether Vivaldi will ever be released, or become successful if it is.
For the past 18 months, Linux Mint has been developing Cinnamon and Mate. These desktops are complementary solutions for users who want something like a GNOME 2 interface on a modern desktop, with Cinnamon building upon GNOME 3, and Mate forking GNOME 2.
Cinnamon's and Mate's development influence each other, and, contrary to what you might assume, it is not always Cinnamon that gets a feature first. For instance, the new Linux Mint 15 release saw Cinnamon getting a screen saver, a feature that Mate already had. Inevitably, however, the development of the two desktop environments is not always perfectly parallel.
The future developments listed in these sources include support for GStreamer-1.0 in Mate's multimedia tools, which is necessary because GStreamer-0.10, which is currently used, is no longer maintained.
Similarly, support for GTK3, as many observers have pointed out, is becoming increasingly necessary because Mate's GTK is rapidly nearing the end of its lifecycle. Presumably, too, the change will make porting new GNOME apps to Mate easier, and simplify coding for Mate and Cinnamon at the same time. The fact that this project was suggested for the Summer of Code might indicate that this change is not officially scheduled, but it is hard to imagine it being more than 12-18 months away.
Other major changes could be the creation of a plugin system for Mate's file manager, and support for the ePUB format for e-books for its file viewer.
By contrast, hints of Cinnamon's future are rarer online. Probably, though, the introduction of desklets – or desktop applets – in the recent 1.8 release opens up a way to enhance Cinnamon with small but useful additions.
Recently, too, Linux Mint project lead Clement Lefebvre has blogged about changes he would like to see in Cinnamon. Lefebvre proposes more flexible tiling of windows, using KDE as a model, previews of ways to manipulate windows while dragging them, and possible techniques for moving and resizing multiple windows.
Lefebvre began his speculations by suggesting that Cinnamon should "let people compose 'views' out of multiple workspaces they can place and resize on the screen, to a very simple static side tray that you could pull from the side like you pull your top bar in an Android phone, and in which you could place a window of your choice."