I would expect it to remain the same external size for now
http://www.netstor.com.tw/_03/03_02.php?MTE3
http://www.magma.com/roben-3tm
Whilst these aren't Apple Products they do allow to easily rackmount mac mini and add PCI-E slots / Storage easily.
These can so some way to regards replacing servers, especially with the reduced capabilities of OSX Server now.
These are fairly new products and would expect them to talk with Apple before developing and launching.