Given the economies of scale, in my opinion I expect the following:
1) A "retina display" quality IPS LCD touchscreen, with resolution as high as 1600x900.
2) More memory, probably 512 MB to 1 GB RAM and up to 128 GB flash memory.
3) A much faster A4 CPU (possibly as high as 1.5 GHz clock speed) or a new dual-core derivative of the A4.
4) A much faster PowerVR GPU that can decode 720p video at full resolution.
5) Cellular-enabled versions to offer your choice of GSM (AT&T and cellphone carriers overseas that use GSM) or CDMA/LTE (Verizon/Sprint) support.
6) Will add the same cameras found on the 4G iPod touch.
Unfotuantely, this wish list isn't coming for a while.
1) "Retina Display" means that it's a higher resolution than your retina can discern. Now, there's some wiggle-room as to what PPI that actually is (especially as you don't hold your iPad as close to your eye as you do your iPhone), but the resolution needed to be a "Retina Display" would overwhelm the current gen CPU/GPU's.
2) Maybe we'll get the 512MB RAM from the iPhone4. It would make sense in the iPad, but I was shocked when it was not added to the iPod touch. 128GB of flash is still too expensive (given that the price/capacity curve is roughly linear for flash, unlike hard drives).
3) I agree that the successor for the A4 is coming, based on the Cortex-A9 architecture, potentially dual-core. It seems like it won't be in the iPad refresh if the timing of this rumor is true - but maybe Apple/Samsung are ready to go with these chips. Faster than current (1GHz) is unlikely; it took a lot of tweaking to get to 1GHz and would result in a drop in battery life (all things staying equal).
4) If Apple is making only one version of the A4, then it is unlikely to ever include a different GPU. The PowerVR SGX 535 is integrated into the A4; a GPU upgrade will likely come with the A4's successor.
5) It does not seem cost-effective to make (and stock) two lines of the iPad with different cellular boards (i.e., one GSM and one CDMA). Alternatively, if you had one wireless chip that could do both GSM and CDMA, this might work - but I would expect it in the Verizon iPhone4/iPhone5 first.
6) Doubtful to see a rear camera; the iPad would be pretty unwieldy as a camera. If it did get a camera, it would likely get the iPhone4's camera, as device thickness isn't an issue (in comparison to the iPod touch).
What do I think is coming?
1) 512MB RAM
2) Gyroscopes
3) Front camera
4) HSUPA
5) iOS 4 - and I'm hopeful that the iPad refresh coincides with iOS 4.2's release
Next next version:
1) Apple "A5" chip (faster, dual-core, better GPU)
2) Higher res screen
3) 128GB flash option