Many great ideas that I won't repeat.

A "nice to have" would be a digital DJ-oriented half that has multiple line-outs for both monitoring in headphones and the master signal for speakers. USB ports to connect midi controllers and an external harddrive of music files. Would all come down to the quality of the apps, but maybe there already exists something for android. Something like the Urbanears "Slussen" connector/app for iDevices. Would be fun.

I think I would generally go for a more expensive half that does many "necessary" things moderately well then a multitude of halves that just do one thing really well. I can't see myself owning more than 3-4 halves: 1 for everyday use (that did storage, battery stuff), and a few for application-specific use like serious photography, DJing, or car use.

I don't think I would do profile-specific halves like they mentioned, it's too trivial a use-case for me (work vs freetime). I would want to minimize the situations where I was wishing I brought an alternate half with me.