i really like the idea of a 'farm' motif on the island....
build that mansion above the vault for aesthetics purposes...., incorporate the nether portal into it somehow, and try to get it centered over the vault..., but make sure it doesnt make getting to the vault from ground level a hassle
A barn, a silo, windmill, a dock (we have a boat)..
possibly two boats
As for the island itself I intend on using that whole upper area to have animals roaming around i mean i want a RIDICULOUS amount of animals... lol. I want an over abundance of all resources..., so when thinking of the area there dont envision the animals to be trapped in small pens !
We will just let them roam that whole upper area, in conjunction with that however, all perimeter walls need to be at LEAST 2 high, but really 3 high... I had animals jumping out at 2 high!... i think they jump on top of each other