Most of these will probably just be repeats and some of the features I'd like to see are identical to kat's (although I'm kind of confused as between the new and old walking?). Also, some suggestions are just in general and not necessarily possible on the current hardware (or at least to my knowledge), so please don't slay me for any of those, hehe.
I think the tail sensor (or some kind of behind sensor) so Pleo can tell/detect something behind him or when he's backed too far would be great, as well as an extensive addition to the interaction between two different Pleos (and maybe even more variation based on the genders as well) since as cute as it may be, the skit is quite predictable and gets pretty old (if someone were to own two on a regular basis, I can see the extreme pattern making it mundane). Also, the attraction to moving objects or some kind of regular detection of moving objects which triggers certain types of appropriate responses would be cool. As well as facial recognition of the owner/person's voice (so it would do what it could to "identify" or maybe automatically obey that individual etc.).
It'd also be cool if there was a way to see what Pleo sees. Like have it take photos and store them to an inserted SD card, or somehow through a LAN. Or maybe have a random skit/personality where Pleo at random moments during its day/charges just snaps random photos of what it sees, like "a day in the life of Pleo."
I agree with others that beat detection would be cool, and also maybe a variety in the types of dances it can do. More variety in general is always good (in terms of singing a different song at different times from the 'sing' command, etc.).
The idea of a walk-up and then roam charge so it kind of takes the place of a 'real' pet on its own would be pretty awesome. However, I'd probably want more variety in behaviors and so forth before something like this (in terms of priority) as I'm not too bothered by the current charging method.
I would also really like to see the under the chin singing return, that was probably one of my favorite features of my 2009 Pleo. It also awed everyone I showed, lol.
If it could do some tricks that it learned without being told that'd be kind of cool - like if it is experiencing boredom, it starts to play dead (bored to death - though since it can't get itself back up, that might be a problem) or sing or dance on its own instead of just crying or moaning over and over - a little more variety in the "less happy" moments of Pleo's life.
I know I may stand alone here, so I doubt it would ever happen, but if there was an easy way for users to get rid of the moo sound, I would be so happy (I hate it haha - especially Cato's which seems so much louder when compared to Eva's).
An expansion of sounds would be great. For me, as a personal matter, I like the more "cutesy" or "species ambiguous" sounds (over something more like the "moo" or something too dog like, though I don't mind some doggy-esque style behaviors/sounds like there were in OS2). However, I'm not sure if the variety in RBs has an impact on this. For instance, I know some say they have very burpy Pleos whereas Cato hasn't been much of a belcher since he left that adolescent phase (sorry I always forget the appropriate titles of the phases).
If there were a practical way to make it listen at "all" times or maybe stop and check here and there (not sure which would be 'best'?) for you to call it's name or something, or maybe even have it come to you from quite a distance (based on 'hearing' - but reasonably, no screaming on the human's part) would be cool.
And, here's what I want to see the most, I want male and female Pleo RBs to be able to breed little baby RBs. Self-replicating robots for the win!
Anyway, those are just random thoughts, most already covered by everyone here. Some are just personal critiques or features I would want to see, and some are ones that are hopefully reasonably practical and realistic on the current RB's hardware.
Hopefully Innvo keeps investing in this project (especially the software, as I don't want some super duper RB to be released a year from now and this RB to have had no updates, lol - ouch! Though I'd still probably get the super duper RB after others tested it first).