Will KDE ever go in a similar direction as Gnome Shell/Unity?

[Copper Bezel]

I thought Unity was a compiz plugin, not a gnome plugin.

I'm not well versed in the Gnome architecture; I've done some tinkering with plasma, so I get how KDE is put together to some extent. I've heard people talking about Gnome "plugins" and alternate shells, so I assume there's some level of modularity there (at least enough that the whole thing won't have to be rewritten from scratch to embrace a new desktop paradigm?).

Is that not correct?

Not quite, or it's a bit oversimplified, at least. It's not possible to have a "gnome plugin," because Gnome isn't a single application. A "plugin" adds additional features to a particular application (and is usually binary and installed system-wide, where an "extension" sets parameters within a given framework provided by the installed software.) Compiz uses plugins in the role that Gnome Shell uses extensions; most web browsers have both. But a web browser, Compiz, or Gnome Shell is a single application.

Gnome is the entire desktop environment, which is a collection of applications run together and talking to one another. Collectively, they're called the "desktop stack."
Running Gnome means running Gnome's session manager, policy kit, etc., and a list of required components, like the window manager, desktop manager, etc, along with daemons to control settings, manage functions like audio, power management, and networking, etc. Those components can be designed as interchangeable, tightly interdependent, or even combined into other processes.

It's possible to start a session with just a window manager and a terminal, literally just those two processes, and fire up whichever services you need from there. (Fun to play with, and I'd recommend trying it if you haven't.)

Using Compiz in place of Mutter (or previously Metacity) means running most of the desktop stack, but replacing the window manager with an equivalent component. That's still possible in Gnome, and it's how Unity works, but the Gnome devs actually seem resistant to this, and some of those other background services can't be replaced at all in Gnome 3. If you're using Gnome's settings daemon, then (AFIK) you're using Gnome's power manager, network manager, audio device control, etc., and Gnome Shell's panel will only talk to those components. In Gnome 2, it's possible to replace all of those things (including the window manager) and still use Gnome's panel, or settings daemon, or policy kit, etc.

When we're saying that Gnome 3 is less modular, it means that the components that make up the desktop environment are more tailor-made for each other and less amenable to being replaced with alternative applications. So it's nothing to do with plugins and extensions, which just alter the behavior of those individual apps.

[weasel fierce]

In a way, the KDE guys took the leap "ahead" a while ago. Gnome has taken it's leap now. I think it'll be a while before something radically different crops up.

I'm sure we'll see plenty of polishing and bug-fixing, and new features being added, but I can't see either environment needing massive overhauls currently.

[Glenn nl]

They are more or less. Look up Telepathy-KDE.