That's a really great question.
I would bet
The most likely scenario is to keep the cost of the system at $249
and advertise Wii Sports / Wii Sports 2
Plus A Controller / Nunchuk and a Motion Plus
They would most likely release something like this in Christmas 2009
This way the new peripheral is already with every system and gives incentive for 3rd parties to work on Motion Plus Games
Predictions For Last Full Week of Sales in 2010)
WII - 80.35 Million
360 - 43.88 Million
PS3 - 41.40 Million
More terrible predictions coming Jan. 2011!!!