i think set buildings would be nice, and then have a few people that you know could build well be contracted out by players to make things such as castles. i dont trust many people on blockland to make anything of quality... =(
this.
As you may already know, I love Medieval RPGs. You should make it so that the players work together to solve quests. The map should be large with many different sections to it, such as villages, maybe a winter-like area, farms, lakes, etc. Something like Eole Island except much bigger and medieval-themed.
The mod should be realistic, meaning that to stay alive, you would have to harvest and find food and water. There should also be animals in forest-like areas that can attack the players. Players should be able to form groups that to join, you would have to, for instance, kill 5 animals or find food for everyone that night.
Lanterns should be placed in appropriate areas; it's always bad when you cannot see where you're going because nighttime in the Day/Night cycle is so dark.
As Monkeyboy suggested, there should be castles that could possibly be bought from a group for a certain amount of money/other items. It would be nice if there was a trading system also. Large groups could split up into pairs and do quests together to bring back items for the group (If you decided that if a player completed a quest, they would be awarded money or items.)
If possible, there could also be a clothing system so that you start out in regular clothing and can progress to get armor, etc.
Also, with the map, I am hoping it can be similar to this style:

There are tundras, canyons, mountains, bridges, villages, settlements, lakes, etc.
I would LOVE to help with the project, as I know that with you in control, it can possibly be the best medieval RPG in Blockland.
If you need any more ideas, I'm full of them.