It doesn't make sense to create a stream under /username and create the first thought under /inbox (that's what happens when you visit inbox first and haven't made a stream yet.)
Firstly, you don't have to create a thought in the Inbox first. You can create a stream and immediately start adding thoughts to it.
However, what I suspect you're asking for is the ability when "moving" a card from Inbox to another stream to be able to create a new stream right there as part of the process and this is planned.