My dream player is a X5 with an empty mSATA slot (so that I can add my own 1TB mSATA SSD for $530, 500GB for $300 or 250GB for $170). Or X5 with 4 (four) microSD (TF) slots.
I am sure you have seen the poll result (wish list) for X7's features. Ability to support large storage was the second most wanted feature from over 100 Headfi votes. Maybe you can set up another vote and ask how much storage people would "dream" of. In my humble opinion, if you have lots of (high resolution) music, 256 GB is a must, 512GB is nice, 1024GB is a "dream". One of the selling point of X5/X3/X1 is high resolution playback, correct?
MicroSD / TF cards are still too expensive for too little storage. With that said, if you equip X7 with
4 microSD slots, I will happily fill them all with 128GB cards now. ~500GB of microSD will cost me over $480 now, while ~500GB of mSATA SSD costs only $300 currently.
mSATA SSDs are getting even bigger and cheaper because they are commonly used in tablets/laptops. Currently a 480GB (Crucial/Micron) mSATA SSD can run for 8-13 hours in a Rockboxed iPod Video 5.5th generation with 850 mAh battery, so it does NOT need a huge battery or a super thick enclosure:
I know this would be a challenge but the idea of a mSATA disk is great. Your normal user can use the TF slots, and your powerusers can add a mSATA DIY style.