I for one would like one...
Now - it's not a given by any means - if you look into stuff at first, you'd see that Colosseum's and XD sales were although ok, nothing outstanding. The point on the storyline being good is a subjective one as well - some may disagree (although generally from what I've seen it is liked, bar the question marks that arise from it).
But I'd say it's possible, and nothing more than that for now. After all - the company which made Colosseum and XD (Genius Sonority) also were entrusted to make the next Wii game in PBR - so it's quite likely that they would make a new game for the Wii if it happens. And one thing you may notice about Genius Sonority is that they like to reuse stuff. XD was a direct sequel to Colosseum (despite them saying it wouldn't be early in development), which used the same region with some additions, and the same concept - and also reused the same music for many a place, or shifted music to play at different places (IIRC, Colo's 'Map screen' music became XD's 'outside Orre Colosseum' music, for instance). Also, some of the music from Colo/XD got remixed in PBR - IMO if they made a Wii RPG (which IMO can easily happen), then it's quite possible they would do an XD sequel.
Of course, if they did that - I'd like to see improvements. Less plotholes in the storyline for one would be nice, or at least some explanations (after all - what DID happen with Wes? =/). On that note, I'd be very glad if the protagonist wasn't some midget red-haired kid with the inability to change facial expressions often but someone more interesting, like what Wes was (teenager who worked for a criminal gang and blew up their base? HECK YES!). =P
Wild Pokemon could also be improved on - in XD they did bring them in, but using cake ('Pokefood) and all? Not that I have anything against cake, but it's nowhere near as fun as how the handheld games did it. Routes would be nice, but I'd doubt to see it work just like the handheld games, tbh. Nevertheless, I'd like to see routes as well.
Either way, this ain't THAT likely to be made as of yet IMHO - just possible, and not for a while anyway (especially as they are releasing Platinum first, and will focus on selling that for the time being - marketing strategies FTW). And there is nothing in the way of 'hints' towards it either (such as what the possible GS remake has going for it ATM). But it's not to be ruled out.
One final thing, and I think I speak for most of us here - IF an XD sequel/Shadow Pokemon game was to be made - it NEEDS Miror B. 'Nuff said. -_-