Allow all members of Amazon Household to cast to Echo
Currently, the Echos have to be linked to an account (which is fine), however, only devices logged into that account are able to cast to an Echo from Amazon Music on iOS/Android.
I have 5 Echo Devices, all linked to my own account, though, I have an Amazon Music Family Subscription and my account is also the Primary Account of Amazon Household.
My suggestion is that any other accounts linked by Amazon Household to the account used on the Echos should be able to cast to those devices too... in fact, my suggestion is that the other Adult Account in the Amazon Household should have the same permissions to the device(s) as the account registered to the Echo.
I definitely feel that the use of Voice Pattern Recognition (i.e., Alexa's ability to understand who is talking to her) coupled with the above suggestion would offer a far more fluid feel to the Echo line of Devices. It would remove the feeling of "ownership" of Echo Devices, and allow the household to feel like a household.