Power consumption is indeed critical. I've used some other modems that I had to switch off via a transistor since their standby power usage was way too large. LEDs are great for debugging, but just suck power. (One board I used was drawing 80 mA without even having the modem on!) So whatever happens, having a low power sleep mode is critical for me. (With the transistor and a custom Arduino, I'm now down to 3.5 mA in sleep, which is decent.) So what I'm saying is make sure the modem is really off when not needed, not some half-powered standby. If you want LEDs, put in a jumper so I can turn them off.
As for powering it, LiPo is what I'd use. However, the USB battery booster packs are rather convenient for users, since they are easy to connect and only go in one way. Makes testing easy, too, since I can just plug in +5V or into the computer. You obviously need a connector, so whatever you pick, it should be relatively common and easy to get. Including the other gender in the box would be great, so I can just make a cable, or at least include a USB to whatever you select.
Package wise, all I really care about is durability. If you use the WLCSP, it must be encapsulated very well. The LQFP just seems easier, though, IMHO.
Antenna: you'll likely never get consensus on this, so maybe let people pick from 2 or 3? I want something that takes up as little space as possible, but is omnidirectional.