Well my vote is for inside the avatar - I'd be concerned about the impact on layout when you make the user info area that much wider or higher, especially when viewing on a mobile, but as Brogan said, use css to make it look how you want to.
What if you want it in the postbit, and the theme is actually green?
As stated a couple of other times, leave it as it is, and if needed, use a simple template edit or css to change the location of it. This quote right here covers pretty much everything needed to understand why....
True - but for such a useful feature it's a shame it is so discreet. My member demographic is quite old & many with dodgy eyes (myself included!) plus they are used to the big icon shown below. It would be a shock for them when we move. I appreciate the new feature can be altered via CSS but would it be a big job for someone who knows what they do to create a online/offline button like this?
There are probably a lot more forums that have at least 1 member with a green avatar than there are forums with green themes. Also, for members customizing the theme it makes more sense for them to have to change the indicator than it does for someone who wishes to use Xenforo right out of the box and might have members with a green avatar.
Subjective of course. My members want an online or offline banner & even a location banner (helps 'localise' advice given). We also have a drop down allowing users to message the member, read their other threads or posts without having to go to their profile page. I call that enhanced functionality handled in a simple & elegant way. XF strikes me as too bare-bones in this regard but XF seems solid & well supported with plenty of add-ons & customisation & the best of the bunch for our needs which is why I've coughed up my dough for the licence & am commissioning add-ons to get us to our starting point.
As a non-coding admin my preference is going to be able to have the choice to have these things configurable via the ACP as I wouldn't know how to do a CSS tweak.