Let's see, where to begin:
Full compatibility with normal versions of mPlayer, both for music and video, alongside preferably the ability to easily view photos or read e-books. You might think it goes without saying, but it is necessary in a replacement for the menu.
Setings data including, preferably, TV-out, a battery meter, and LCD fiddles. Possibly also a startup config bit, to allow you to add various widgets and/or checks to the loading of the console. For most, I imagine this would be fairly sparse and untouched, but it would be useful in terms of manually telling the unit how much you wanted it to write on start up and where to, for example. possibly also the ability to switch NAND between read-only and normal in the menu. Overclock settings, preferably with different rates for inside menu and outside. An extension of this might be to allow the linking of some programs to particular clockspeeds set before the menu were to close, though I don't know how many would actually bother to do so.
USB link which works, with any luck.
Then on from functionality to more appearance factors:
Possibility to have animated icons (probably use gif or an array of images in some other format), with scriptable pausing, looping, or continuing from a given frame, preferably linked to being moved in any of the four cardinal directions, or along a path template or some such.
Custom sizes of icons, alongside position.
Possible animated backgrounds (I'm thinking plasma things, but I'm sure you could simply set it to use an animating pic for simplicity).
The ability to script icons to different menus, programs, or potential future tasks. Some automation of this might be handy (Mocca style), though it'd be sensible to leave that up to the theme designer.
Might be useful to be able to use a directory of scripts in the root dir where it could look for custom "per-SD" things. Alternatively, read an SD card's ID when inserted, and then refer to a set of scripts stored on the Nand.
I think that's pretty mcuh everything I can think of, although obviously a lot of it is pretty superfluous; but in terms of an ideal replacement, with freedom of skinning and the capability to look very very snazzy indeed, that'd be about what I'd be after in one. Think, everything the current one has, with the capability of being extended through scripts, and a very versatile skin system...
