Do you enjoy doing automatic farms or any kind of redstone machines? A lot of them end up taking up a ton of room, and there is a lot of practical utility to a lot of them. I also think it is a whole lot of fun, to make them look nice after I've built them.
Have you considered an atrium (could double as a small farm/tree farm and still look good), a music room with a working grand piano (cubfan has a design), a swimming pool (that could also put items into a storage/sorting facility), rooms just to look at the sunset and sunrise, and/or rooms to take up fortifications from (with keyhole windows)? And I would also add some sort of super-smelting facility, which could take the shape of a kitchen, or even a more modern workshop.
Of course a major part of designing a base is how it looks, not just practicality, and to that end I would also add a few small but similar garden rooms, and maybe a room where I could put tokens from the memorable things you do on the server, and of course a giant entry just to wow any visitors.
I'm about to build a castle, and I also intend to fill it with rooms, and I would also like to make every room practical, but it also has to be interesting to look at in every room, or I will quickly loose interest and probably won't finish, lol.
One final thought... maybe if there is a spawner near by, or if you feel like making a mob farm you could have them all come out to one spot and walk up towards the outer/fortified wall of your base so that you could spend all night fighting waves of mobs for items. I bet you could even give them fall damage before they come out to make it easier.
Hope these help ^^