There's only a few things that can change… form factor, charging, or manufacturing.
Could make one smaller / thinner / wallet-y
Could make them with charging using the watch charger and recharging batteries
Could make them more waterproof or less "amendable", or colours, or improve range/transmission
I think they are going to have a very hard time improving these given the standards and power requirements. I'm personally expecting that they don't change the original, but offer a few more choices like a wallet option, or some sort of car option. Would be interesting if they can charge via motion, like some watches do - think bicycle; enough movement by simply riding. They could also offer NFC type of scenario where there is no battery, but I'm not sure it's in a usable state for tracking - just too much transmission power needed.
But they're pretty boxed in because of how good it is, and the size of a battery needed to run it.