My perfect player would be a sansa clip + with the following changes:
Built in amplifier
Larger battery (2 - 4x capacity)
16-32gb onboard storage, with two microsd card slots
Nice ideas you have. I'm more of a MP4 guy myself, but sometimes when I have new music or an audio book, I've wanted to have an portable music player with really good sound quality. I really like the larger battery idea. When I buy phones, I usually by a larger extended batter of the net. I would take a look at a music player w/ a big 3500 mAh battery if any company actually would take a chance. However, there would always be the one's giving it bad reviews for being too big or too heavy. Sigh....
I like your idea of double microSD card slots too. There may not be a market for it, bit I'm in a unique situation where I have multiple electronics devices that I've bought over the years and could actually use that feature. So have a half dozen microSD cards floating around that I keep different types of files on. I am constantly ripping movies to watch on my commute, so try to keep them on a separate card since all that copy and delete is wearing my card out. So I could use one slot for the card that I'm regularly taking out and plugging into my computer and leaving the other slot for more permanent files like audiobooks from Audible.
I don't know how many people would want two card slots in their music player. Design wise, they could have both slots accessible from the outside of the player, just labeled something like A or B. Another idea may be to have one slot underneath the battery or battery door like a smartphone and the other on the outside of the player.
One problem may be how long it takes the music player to update with 16-32gb internal memory and two card slots. I've had some MP3 players that took a long time to turn on because it would update the media library every time. If you have two 16gb cards, I wonder what the boot time would be on a portable device like this.