1) A molerat farm and a glue factory. Molerat farm produces molerat meat. Glue factory produces adhesive from vegetable starch ingredients (farms) or from molerat fat if the molerat farm is present (synergy!). Meaning that if both are in the same settlement it just makes adhesive without consuming actual food items.
2) Minutemen specific guard posts that work together across multiple settlements. If a town comes under attack first the script checks immediate supply routes (direct link, not multiple). If there is a minuteman guard post at the linked town then it sends its guard(s) to assist in fighting off the attack; said units will teleport to just outside of town just before the enemies beam in as well. Example: Abernathy Farm is scheduled to be attacked by raiders. There is a supply link to Red Rocket. Red Rocket has a minuteman guard post with two soldiers. When the player arrives to defend (or the odds are simply autocalculated if the player ignores it) then the two troopers from Red Rocket will also be present to defend Abernathy Farm when the raiders arrive.
3) A shop that sells automatron/robot mods.
4) Recycling facility (industrial) that, over time, breaks down junk left in settlement storage down into its component parts. Like the contraptions recycler but doesn't require any power. Can also be instructed (via holotape) to break down weapons, armor, ammo, etc, also like the contraptions devices.
5) Guard animals (military) stations. One human (or robot) required, spawn +1 guard animal per level of development (max three). Dogs are obvious choice, or could simply increase the level of the single animal instead of spawning multiples. For example, at L3 it could spawn a trained Yau-Guai to help defend your settlement.
6) Bring back laundry and shower facilities, please! I like having clean settlers.
7) Clean room (advanced industrial) with a radiation removal arch inside. Increases settlement happiness and cleans folk of radiation damage.
8) Hunter's Hut (security). Lower than average security bonus, but also produces food on the side. Example: +5 security, +4 food at level 1. Food are all meat types, obviously.
9) Vertibird Platform (3x3 advanced security). Requires a LOT of resources, but will spawn a NPC vertibird aligned to your faction to pulverize anyone who'd dare attack. We'd be talking +100 security plus a flying death machine with a minigun in real-time combat. Fast travel from any settlement with a VBP only takes 1/5th the normal time (one way only, though).