I don't see much a fit for payment features in fitbit products. They should probability focus on iterating the hardware and experience regarding the "sport watch" idea. Their strength is also the simplicity of the product.
EDIT: interesting examples provided in the replies, thanks. To buy things while running without having anything with you is a good thing, also if this technology allows to, like, register in a gym or alike and enter without any other token.
I'm not saying I want this, but it would be possible:
Imagine your access card for the gym doesn't provide free access to everything, but every time you enter the sauna it bills you one dollar, or physically walking into a class bills you the individual class tuition. Unlocking the racquetball court door costs three dollars. Then after your workout you walk up to the vending machine to get your bottled tapwater or fancy water or whatever and pay with your gym access card. Inevitably there are not enough treadmills for peak times, so something is set up today like signup sheets and time use limits, but crazy as it sounds you could pay per use or bid to reserve, high bid on treadmill #7 from 6:00 to 6:30 tonight is two dollars, wave your magic token thingy here to give you the new high bid at three dollars. Gamify things such that every 100 miles on the treadmill means a free run next time. Insert anecdote of people hooking up their fitbit to a cordless drill to get 100 miles per day so as to get free treadmill time.
EDIT: interesting examples provided in the replies, thanks. To buy things while running without having anything with you is a good thing, also if this technology allows to, like, register in a gym or alike and enter without any other token.