While it'd be nice to have your entire collection in the highest quality on an iPod, I doubt there's much market for it.
My music collection, which is large enough that it shocks everyone I've shown it to, is currently 189 GB. Yes, I know there are people with larger collections. Yourself, obviously. But you're in "the 1%".
Now, about a quarter of my collection is FLAC (rest MP3/AAC/Vorbis) - and the average bit rate across the entire collection is 382 kbps. 17,481 tracks, seven solid weeks of playtime.
For a portable player, I've got no issue compressing down to ~130 kbps AAC... which is 34% of the size. The total size of my collection at that bit rate would be 64 GB - I wouldn't have even hit the halfway point in terms of capacity.
The market of people who even have enough data to fill one of these things up is small. The market for people who have the library, and would actually want to carry it all in their pocket, at all times - is even smaller. And those people likely have got a portable 500 GB drive already.
I wouldn't bet on seeing higher-capacity PMPs for years to come, when the price on storage has come down further.