Presumably Yubico's upgrade path is to tweak the form factor slightly so they can fit more than a few kb of memory into the thing. I know that it's possible, I can buy 50GB flash drives in the micro yubikey form factor, the ones that are just a rectangle of plastic that fits in under a USB-A port's tongue, and they only cost like $10. So it's probably just something that Yubikey needs to design into the next gen of keys, and I suspect it won't make them cost much more than $5 more than the last gen.
