Auto DST is in the plan but will probably be hardcoded due to overall code size.
Don't forget that some of us live in places where we don't feel the need to confuse people and computers by fiddling with the time twice per year 
Your design is nicely done, but I would encourage you to "Think outside the (Arduino) box", and not limit yourself to a standard shield size. Especially since it's limited you to a relatively small 4-digit display. If you want a bigger display, just design a bigger board, and don't worry about the fact that the outline doesn't match the Arduino.
If you do go with a bigger board, consider adding pads to support a 6-digit display. And regardless of board size, a jumper to select 12/24 hour display would be nice.
Ran