1/4 inch jack that works nicely even with 600ohms headphones along with 3.5 mm jack that works nicely with sensitive IEMs would be really great... Not sure if it's technically doable though-- hope it is!
To reduce the size, I echo the request for removable batteries. A large removable cellphone battery like Xiaomi Note (3200 mah) would make the device lighter-- but still give a cheap and convenient option to easily double or triple the playtime if needed. I think they sell their original batteries for like $10 or so. Or, would it be possible to run via microUSB? What I mean here is, being powered efficiently by a powerbank connected to the microUSB. I think typically, the powerbank charges the internal battery which is then used by the device. I would think is very inefficient and results in a lot of power loss. If it's possible to power the device directly (without going through the internal battery) from a 10,000 mah pack, that would solve the balance between battery life for long trips and portability when you don't need it.
To further reduce the size and cost (since a lot of the requests are likely adding to it), why not just put enough RAM for the OS and future upgrades-- and completely rely on external storage for music? I think it's better and more flexible to have double microSD slots than to have a single slot + 64GB/128GB internal. I would personally prefer full SD even if just a single slot (though dual would be awesome) since that tends to be cheaper, more durable, and have bigger capacities.
To further reduce size, cost, and power consumption, I prefer a similar physical layout to the X3/X5-- that is, physical buttons and a decent screen. I personally don't care for a full sized screen since I think the device should be true to being a music player... but I guess the trend is going to full sized touched screens. :-( Admittedly, it's one of things I dislike about the DX100.
Guess I'll hold off from purchasing the DX100. Hope X7 is for early 2015 :-)