1. Manufacture
Ever wonder why manufactures choose to have base models? Because most factories are designed the produce hundreds of thousands of the same phone. Sure, producing hundreds of thousands of individual components wouldn't be much different, however of each individual componenet (battery, storage, camera etc) There would need to be multiple different versions for true "customization." And if multiple partners get involved, and there are different partners with different model, then suddenly you have the issue of making components that work within each model
Verdict: Unlikely
In all honesty, I would love for it to work. It would be an awesome phone to have, being able to customize your phone, as evidenced by the excited build-up towards the Moto X's supposed customization. However, at this point in history, it is unlikely to be accepted by manufacturers. We can hope, but always take these things with a dash of realism. Its ok not to get your hopes up, especially if the PhoneBlok ends up being a reality. Plenty of time to be ecstatic then! Thoughts?