When I first saw castle Dawnguard, I was like holy crap! This is the best castle I've seen in this game. I realized that you would never be able to properly siege/defend this castle, and that bummed me out. Could someone change the kill the Dawnguard quest to an outright siege somehow? This or a quest for the Dawnguard after Harkon is killed..
Now for the siege of Fort Dawnguard..
I was thinking put up archers on the castle, add some crazy ass siege weapons, et cetera. I would just like to see its potential used instead of having this awesome castle just being aesthetic.
As for defending it..
There could be a short quest after Harkon is killed. Have a new member of the Dawnguard walk up to you when you enter the castle. This person is looking to join, and he/she has a band of people to help, but then they ask that you accompany them to the roof. Once on the roof they take you to the battlements and say that they've spotted a large group of vampires coming. Then immediately vampire attack, and people go to the battlements, which can include you. Once you decide whether to stay on the roof or go to the ground level, the assault will begin. Then LET THE DAMN CRAZY ASS SIEGE BEGIN! : D During this battle Isran would be killed and you would become the new leader.
It'd be a pretty hard thing to do, but I think many people would like to see it done. It'd make the Whiterun/Windhelm/Solitude sieges look like crap.