The Hard Drive bay at the bottom across from the psu. It would be nice to see a 2.5 vertical mounting option so that it can be possible to mount more then just 2 disk and have them hidden from the main area. With the space that is available, you could have a few vertical or horizontal 2.5" bay drive options.
Also it would be nice to have an option to cover "in the main area to the right of the mobo" to hide disks or rads etc.
Also in the main area where you already have the section for x2 2.5 inch disks, it would be cool if you could just include the holes on the bottom plate, so x2 "good looking" disks could be mounted and displayed and not covered.
Another thing I would love to see is a mountable bottom plate that holds unusual Reservoirs. such as the "Tvirus" looking ones from Frozen CPU.
It is also nice that there still is a little bit of room at the bottom of the case between the psu and the hard drive bracket, This would be a great place to mount an ugly water pump. however i would like to see some options in the main area on the far right of the motherboard, whcih (if you include my recommendation above) would be able to be covered.
and lastly but not least,
Slidable mother board trays and back panals for different kinds of mother boards, I would love to use this case to build in a supermicro mother board that does not have its back panal in the traditional layout.
Or at least put some holes on the back panal for better "cable tie" cable management options.
I hope this helps.