I don't think Apple is going to stick with a form factor remotely similar to the iPod's. Though Apple isn't known for succumbing to the bandwagon effect, both Rio and Creative, in their Nitrus/Karma and MuVo2/Zen Xtra, respectively, have entirely different form factors on their devices. Each of apple's product lines (e.g. iMac, eMac, G5, Albook, iBook, iPod) fill a distinct niche, and Apple strives for virtually no overlap. Each of these product lines is also visually distinct.
Thus, I doubt that Apple will try to preserve the current iPod's form factor, especially when a smaller HDD platter size allows them flexibility in this respect.
Lepton: I truly doubt that Apple would put the controls on the back, esp with the touchwheel. Ever tried to just "turning" the touchwheel a full circle with your iPod flipped over? It's not easy. Also, how will you hold the player? The screen fills the front, and the wheel is on the back. You'd need duct tape on the sides to keep the player in your hands, and keep it functional.
Alternate form factor: More plausible (IMHO), but personally, I wouldn't like the feel of a tiny touchwheel.
My own little twist: Take away the symbolic iPod touchwheel. Make the miniPod square, height and width being the same as the crrent iPod's, or maybe even a bit smaller. Then, make the top-right corner a three-way rocker switch (up-left, down-right, press-in) That way, you don't have the poor aesthetics of a jog wheel, yet you have something that lets you navigate a smaller library just as effectively, esp. when coupled with something akin to the current iPod's variable-speed scrolling. Have another three-way-rocker on the upper-left corner to handle << >|| >>.
Have a variant of the iPod dock for the miniPod, maybe something less geometrically simple (minimalist), and rounder, for that more-fun-and-less-sophisticated look.
That's my current whim.
P.S. - If HDD, I'd go for the Cornice drive. Cheap enough to make a price point diff from the current ones, and still large enough to be "high-end ultraportable". Of course, you could go for 512MB flash. Makes the device much thinner, extends battery life, and basically makes it skip-proof.