Possible things they are doing:
-Different maH battery
-Gingerbread
-Skype being re integrated
-changing out a bit of hardware(better bat life)
-coming up with 4g pricing based on the phones "average intake of data"
I don't think they will swap out the battery. That is too cost prohibitive, and supposedly the short life issue was fixed by the recent firmware update. That would be great if they did swap them out though, and the most efficient way to do it would be to ship them to the stores and trade them out at the point of purchase, instead of taking them all out and replacing them in the warehouse before they go on sale. Anyway, they would not be able to make it that much bigger because of size restrictions. Seidio makes a 1600 mAh, which might be as big as you could fit without having a hump and new battery door. Not sure if 1750 would be possible or not, but if it was I'm sure Seidio would have done that instead of 1600.
Gingerbread might be a possibility, but why not just have an update in-store when purchased, or shortly thereafter if it's not quite ready yet? It should not cause further delay on the phone's release.
Why would they spend the time taking Skype out, and removing references on HTC's website, just to re-integrate it right before release? I think there was some problem and they decided to scrap the integration altogether, or at least to wait for an update sometime in the future when they correct whatever was wrong with it (assuming they can).
They simply would not change any hardware this late in the game. It would have to go back through the FCC to get approval again. That means more time and $.
I think they will come up with 4G pricing over the next few months, not that they are hustling to figure it out now. It is not widely enough available yet to realistically charge everyone for it at this point.
Any change they would be making to the device would most likely be software related, imho, and if anything at all (there might be nothing else going on now after the final firmware update except for shipping to the stores). For example, adding HTC Watch or OnLive (features shown off on the recent HTC Flyer tab at MWC), or integration with HTCSense.com.