Well, now we know why they've been silent on the firmware updates for so long. Now that the HD500X is out of the bag, maybe we can look forward to a firmware update in the not too distant future.
Here's a list of amps that I'd like to see make the POD;
A 6505/5150 lead channel would still be a good idea for starters. As awesome as the SLO Overdrive model is, it's authenticity makes it problematic at times. Turning up the gain adds low end, but at the expense of tightness ( true to the original apparently. I thought it was me, but doing some research and L6 really nailed the nuances of how the EQ and Gain controls interact with each other). It's also tonally different enough that it would be a welcome addition.
A 6505+ lead channel model would be great too. It's darker and bolder than the 6505, and a bit more aggressive sounding. It's different enough from the original 6505 sound to warrant its own model. It's a great sound for any hard rock or metal style and is a staple of the genre.
Bogner Ecstasy Blue and Red channel models would be spectacular; If you haven't heard the Ecstasy, listen to some clips right now and tell me we don't want/need these two channels in the POD! Not to mention that L6 modeled it on the X3, so no reason they can't give it the HD treatment.
Same goes for the Diezel VH4; Channels 2 through 4 need make an appearance. They were in the X3, and it's another FANTASTIC sounding high gain amp. A staple of high gain tone if ever there was one.
A Diezel Herbert needs to make an appearance. Awesome amp. 'nough said.
A JC120 is high on the list, only because people are still clamoring for it. I was a big proponent a while back for it, but I've come to really enjoy the fenders, voxes, and the SLO 100 clean. I really could care less if it ever made it onto the POD now, but I know other people can't live without it so it needs to be on this list.
Other great amps that I really wanted to see on the POD, but have since stop jonsin' for because I've learned to live with the current line up and have come up with patches that sound close enough;
JCM900/2000, Silver Jubilee, and Mk IIc+. They're great amps, and would be great on the HD, but I personally can live without them now. If we got the previous 6 I listed, and no others, I could not only live with, but thoroughly enjoy my POD HD Pro from here on out indefinitely.
When it comes to effects, I've come to grips with everything on offer, except the choruses. I find them kind of weak. A potent chorus or two, and I could live with the rest of the FX.