For as a nifty (and maybe useful to many) output other than the existing relay outputs, I'd personally like to see a 3-pin 100k digital pot output header. I mean, interfacing to one is not hard but no experimenters Xbee board I know of personally has one. I guess either the SPI type or whatever could be used, but for an experimenters board (and maybe for more utility in practice) it'd be a "nice to have" option even if it was provided as a little externally linked board. At any rate, just a fleeting thought. As far as anything ELSE, nah...at least not for any other "suggested" bells and whistles. It's easy to over-feature something, but hard to actually provide truly useful utility across the board. i mean, there's lots of Xbee adapters and shields and blah blah, but who wants all that junk all over ones desk when something like your new board would suffice? People like me who work with Xbee on at least a rudimentary level as serial ports or for simple actuation could use this thing. Would replace a lot of modules cluttering the bench!
-All the best!