Guild leaders have been sending us a lot of questions in the Queue and elsewhere about how to keep their guild active during the down time between expansions. At this point many guilds have already cleared Orgrimmar and are either slowly working on heroic modes or stalled out entirely. It's not that anyone is doing anything wrong -- they're not -- it's just the nature of players. After things have been accomplished, why bother continuing on? This is totally a normal behavior and shouldn't be taken as anyone abandoning the guild or anything silly like that.*
So, life is slow on the forefront of raiding. What can you do? Change your game. Make the game not about killing pixels from this expansion, but about experiencing everything in a social atmosphere. I've had to make this transition a lot, and I've always found it works.
End current content raiding
Make an official end to the current tier of raiding and move to other activities. Allow for a small group to form that wants to keep pushing into harder heroics and what not, but don't make anyone feel like they have to go. And don't feel obligated yourself too -- it's okay in a guild leadership position to sit out and just facilitate.
Enjoy the old world
There is an absolute ton of old content out there. How long has it been since you went into Molten Core with a full group? Go and do that, you'll get gear from transmog, achievements, maybe some pets. This is a great time to experience the entire game and not just current content.
More so, this lets you open up your doors to the rest of the server and invite new people in to experience your guild. This does wonders for recruitment, and is a great way to make sure you have a strong team for WoD content.
We're all adults, so be adults
People have fun with the base activity of WoW, sure, but what keeps people around are the friends they make in-game. In past guilds I'd have drinking nights on mumble, where we'd all get together and drink a certain type of alcohol together. This lead to some great discussions and I learned a heck of a lot about gin and beer.
Now I'm just using drinking as an example here, it could be anything really. Just something that communally connects people together. When people are connected together, whether they're running Black Temple again or romping around Battle Grounds, they're having fun.
And at the end of that day, that's what you want in your guild right now. It's not about downing Garrosh for the 50th time. It's about make sure that everyone has a place to have fun, and that you're having fun as a guild leader. Just remember that this is a game, and reinforce that with your guild. Everything else will work out.
*I realize there are exceptions to this paragraph -- clearly there are still guilds that are pushing forward hard and trying new things constantly. But they're the exceptions, and I'm speaking in broad strokes.