It must be obvious there are several competing and incompatible interests in the next version of the plug, I myself nope the plug will be kept to a bare minimum just like it is. What I would look for is USB 3.0 support as soon as possible, not due to the increased speed, but improved power efficiency. Besides USB 3.0 is already fully supported in the new Linux kernel, and essentially makes eSATA and so on irrelevant. Adding wireless would increase power usage and thus decrease interest in the unit for a lot of purposes, several of mine included, though I could certainly see it succeed as a wireless router too!
Beyond USB 3.0 I believe the only interest would be a choice of flash disk size and ram, as for most purposes 512Mb should be enough.