Similar back to Ipad mini/Ipod touch, 1080p Facetime, NFC, 16:9, 3D, Slimmer, lighter, 8mp camera, the rumoured fingerprint-recognition technology, improved smart covers, possible new colour, 10" screen..
I'm thinking about this, too. Whatever I get will be my first iPad and I want it to last a good long time. I'm leaning towards getting the 4. If the next update is this March, then I might be inclined to wait, as that's less than 6 months away now. But if they've decided on a Fall schedule, and the next one's a year away, then I'd jump in now. Considering Apple is pretty much unpredictable now, I'll just have to go with my gut.
As far as what could possibly be updated? I can think of a couple things. As the Phone goes, so goes the Pad. So, just as the retina came after the phone and the A5 (and now A6) came after the phone, we can expect the next iPad to use the in-cell screen tech and go thinner, lighter, hopefully less hot.
Here's another thought, what about upping the screen resolution. "Retina" isn't a technological term, it's a marketing one, and not all "Retina screens" are created equal. The iPhone (4, 4S, and 5) actually have a significantly higher ppi (around 330) than the iPad 4 (around 260). I've heard that at that size, it doesn't make much of a difference since the letters and graphics you'll be looking at are bigger on the Pad anyway... But, Steve Jobs used to say the magic number past which the eye could no longer distinguish pixels was 300 ppi.. the iPad hasn't hit that yet. I would bet they'd love the opportunity to boost the display resolution and advertise the hell out of that. I doubt they'd go that way as early as March, considering they didn't bother to up the resolution of the iPhone 5 but focus on thin/light, but if the next update is October... then possibly.
Eventually, glasses-free 3D. Eventually, haptic technology on the displays (look it up, it's pretty incredible). I was really rooting for haptic this time around but I guess it's still too early. I'm getting a little concerned that with no Steve Jobs at the helm, Apple's lost the drive to really surprise us with something we didn't even realize we'd want. Part of the reason I want to jump in now is as time goes on, I'm afraid they'll actually become less and less innovative. I hope I'm wrong.