In addition to more custom meshes I would love to see more random settlers added to the leveled lists so I don't keep seeing the same one's over and over again. Seriously? Does eveyone in the Commonwealth have a twin? Also, the ability to recruit settlers as a follower would be cool along with a toggle option in the terminal that allows you to switch settlers from essential to non-essential.
The ability to recruit raiders and gunners to your settlement using a successful intimidation would be pretty awesome!
More radiant quests. Use some imagination. Someone seeking refuge from a bounty hunter, a who done it murder mystery, a trojan horse infiltration, settlement wars, civil wars within the settlement.
What are some of your ideas?