Of course it can happen. There's already a third party who released a sixaxis for PS2 where you can configure the tilt to take over the analog stick, and it gets an excellent rating, and I think it even has rumble. There's a nice lightgun for the system, and the buzz, singstar mikes, and so on all work fine. There's Guitar Hero, and Rockband could just as well be released for the PS2. There's the EyeToy, and there's the fact that all of these peripherals are compatible with the PS3 (even if the software isn't always). There's also no reason to believe that the interesting new lightgun developed for Time Crisis 4 isn't comptable with the PS2, even if they may not immediately release software for it.
The question is just how far devs are going to take any of this. A number of Wii games only need the sixaxis functionality to work, and in a quite limited fashion too, so certainly there's a possibility right there. A limited version of the wii-mote doesn't have to cost much and can easily be bundled with software, because that software isn't necessarily expensive to make.
What makes the wii-mote a success is Nintendo's software development, and all the tuning that has gone into making Wii Play play just right. If a controller were to be developed right now, it would still take a while before games come out that support it properly, although third party developers could benefit from wii-mote experience quite a bit.
It's all a matter of wanting to and being ready to. It could be difficult for Sony, as they already have so many things going on and they'll want to invest in the PS3 more than in the PS2, but then again they themselves have always considered the PS2 to be the more direct competitor to the Wii, so who knows. Certainly a new controller doesn't seem too far fetched, but there is already so much going on that right now I think maybe getting software to benefit from what's already there (think new EyeToy, sixaxis, and the GunCon3 controller) could be much more useful. Maybe a rumble clip-on is possible, or small rumble clip-ons that you put whereever you like on your clothing, hands, controller or body could be interesting ...