It's not that I've got anything against the Rock, but I'd quite like the option of getting new familiars through achievements or even payment. Different personalities, different abilities, different styles of assistance and different quests. They could be additive, rather than simply replace the existing ones, meaning you wouldn't necessarily lose out on anything along the way.
If that were possible, it'd also be nice to have a way to see other people's familiars... another level of customisation, perhaps.