The next Airport series of worth (that would be worth upgrading, really) will likely be when they switch to 802.11ac, which is not yet a final standard. It's also called 'gigabit Wi-Fi'. However, it's not likely any of your devices support gigabit Wi-Fi yet either!
I imagine in a year or two Apple (who usually keeps their stuff near the top with wireless performance) will start introducing 802.11ac in their wireless-equipped hardware, and when that happens an 802.11ac equipped Airport Extreme will come out. However, unlike computers and mobile devices, technology on a Wi-Fi router isn't moving all that fast. Basically, the only REAL upgrades of worth are when wireless standards change.
If you are looking to upgrade your router, go ahead and get the Airport if that's what you think you need. I use a Time Capsule (which is an Airport Extreme with a 2TB hard drive built in), and I love it, performs excellently.
Yes, but recently, there have only been minor antenna improvements. Not a lot has really changed as there really HASN'T been any big changes in consumer network technology. Faster ethernet speeds haven't made it into the consumer market.
You MIGHT see an 802.11ac draft compatible router, though. If you have a good router now, you might hang on for that, but it's anyones guess when that'll be. Of course, you'll need to also upgrade your hardware to support that. If you have a Mac Pro or a desktop PC, a PCI-Express card will take care of that. Otherwise you'll need a dongle (USB 3.0, otherwise you might as well stay on N, you won't be able to realize the speeds of ac over USB 2). So if your computer doesn't support USB 3 OR have a way to install PCI Express cards, then you're kind of stuck on N for now.
In other words, if your computer can be upgraded to ac, and you're willing too, then you might hold out for gigabit wifi Airport. Otherwise, there isn't likely going to be a noteworthy refresh where you're really gonna say 'Oh gee I wish I had waited!". Perhaps a small one, but nothing major.
Also, there are already 802.11ac routers available. However, if they are anything like the early Wireless N routers, I wouldn't hold my breath. Early draft routers tend to be poor.
My wish list, is a Time Capsule is wireless ac, and USB 3.0 and/or FW800! (The latter not likely though as it's slower than USB 3 and being phased out)
Then next update will likely be to support the new 802.11ac standard. I don't see Apple releasing a router with ac until they have client devices that support it. I suspect that will happen with new Mac portables (Haswell) in mid-2013.