I think a fun mod would be one that turns the settlement attacks into more of Tower Defense type game. I think it would nice to also be able to manually start an attack[uamd of this type. Here's my idea:
Settlement Tower Defense:
There should be a craftable item you can put in your settlement, that must be placed on dirt. It's a beacon of sorts, and the goal of the attackers is to get to the item and activate it. If they activate it you lose. I envision it working like the "bell" where the attacks want to get as close to the item as possible, and will try and find a path to it if the can.
There should be a way to check if there's a path to the object from the outside of the settlement, if there isn't than the object/game won't activate/start.
Ideally there would be an announcement system for "waves" of incomming attacks. The quest system could probably be used once GECK comes out so it starts a series of misc quests for each wave of attackers and fails/succeds the quest depending on certain conditions.
Difficulty, ideally the attacks would be scaled to level or could be manually activated at higher difficulty.
Rewards, shipments of materials could spawn on bosses or be lootable from the craftable item once the "game" is completed. depending on the difficulty of the encounter the rewards scale.
I have enjoyed using the Settlement Raiding Mod but by default it mostly just a few hordes of bad guys who charge in. I think if they had a "goal" that they were all trying to get toward it would make things more interesting, because as it stands right now they don't always do a very good job of engaging the settlements. IF there were some sort of "bait" or "target" they were actively rushing toward I think it would be better.