I hope that their "something new" doesn't include the Vitality Sensor.
I thought that the Wii was in the least need of a replacement, so I'm actually leaning more towards a DS replacement. A new Nintendo handheld, combined with Sony's idiotic devotion to keeping the PSP around for 10 years, will pwn the PSP and bring everyone back to the good old Gameboy-only days (imho, those days were actually not that good).
If it is a replacement for the Wii, it would likely be one of three cases:
1) It's a console that walks the line between casual and hardcore gaming. This would be a huge kick in the sack for MS and Sony, and Nintendo will dominate the gaming market for the rest of my lifetime (*sob* I don't like any of the Nintendo franchises *sob*).
2) It's a bit of a return to the hardcore scene. Very unlikely, because this makes no economic sense. It would take a huge bite out of MS and Sony, and might actually allow the Wii to extend its lifespan to 12 years or more. They could simply offer both consoles, one for the more casual crowd and another for the more dedicated.
3) They go full out for the casual crowd. This could either be a success so huge that Nintendo can now afford to buy Europe (including the citizens and all their posessions), or flop because people don't feel like upgrading from their Wii (I still think they totally should have pulled a bit of MS and made the Wii slightly less durable). Either way, this option won't have a massive impact on MS and Sony.
Just my non-professional analysis