You could build a lot of things. A few small details, like flagpoles and a marketplace. You could make the village be more alive besides villagers by using posing armor stands. You could also make things like trade quarters, wishing fountains, statues, livestock trading, farms, taverns, stables, a dockside market, cathedrals with note block bells, carts for the markets, notice boards, beacons, a storage hold, signage, guards posts, and wells. I would reccomend looking at the Minecraft: Medieval Fortress Exploded Builds, which has traps for your castle, a castle keep design, village ideas, and a lot more. Have fun with your build! =3
A few guard towers and stations would be pretty useful. As well as a set aside area for places where one could train in different styles of fighting (such as swords or bows). An armory would be nice to put around different styles of armor. Even a small church would suit the atmosphere as well. Even then, despite the sadness, a graveyard would be pretty useful because I am not sure anyone in that world would want to have an infinite life. Good Luck!