Firstly, I could not locate a forum for suggestions for fallout shelter. I notice most people are just posting their gripes with one post per gripe.
I thought I would offer some of my ideas, even though I find it highly unlikely they will be used.
1. Allow a choice of two types of upgrades for the Vault Entrance, an option of either making your door stronger, or add a security station (+1 guard space). But be limited to 2 upgrades total. So you could get +2 Guards, or +health twice, or one of each upgrade.
2. After obtaining the armory, allow the player to 'upgrade' weapons from Rusty, Enhanced or Focused, to the next level above. for a cost of caps based on what level it is. So a rusty pistol could be upgraded to an Enhanced one for say... 50 caps. Or from Focused to Hardened for 300 Caps.
3. Add traders that approach the vault door to sell to the player. (Armor, Stimpacks, Radaways and Weapons.)
4. Add slavers that approach and sell slaves (that you free and make dwellers).
5. If the player has created a vault at least once, allow an option to skip the tutorial and build from the start as you please instead of forcing the current guided order of the first rooms.
6. Add a new room, Security Station, Allows placement dwellers in this room to be Auto-Guards, when any room on the same level is attacked by raiders, rad-roaches, etc, the guards automatically swap with the current dwellers in that room. Expanding the room allows more dwellers to be stationed and swapped. Upgrading increases the run speed of both the fleeing dwellers and aiding guards. "P" room for happiness reasoning, but offers no 'gains'.
7.If a dweller in the room with dying raiders has a high enough luck, they should be able to obtain one of the raiders armors or weapons after the battle, or maybe even just some caps. I would hate to think my dwellers don't loot the bodies before tossing them aside.
8 If two dwellers are in the Living Quarters and are ready to 'get it on', have a window pop up and ask if you want them to have a baby. If you choose no, they will not try again until you remove them from the room and put them back in.
9. Make the Surgeon gear help Intelligence.... why in the world does it not? Its supposed to be worn by med bay peeps (assumed by its name), right? My dweller dressed like a surgeon is working the water plant... seems odd.
10. Add Nuka Colas which you can only get by finding them in the wasteland. When used by a dweller, they boost their current happiness by 5%. They would be fairly rare.
11. I really hope more encounter variation for the wasteland exploring will be added. Currently, I find about 12 fleeing slaves all shot in the leg on one trip. Seriously, how are they ALL shot in the leg. Variation is nice, even if just changing a word or two here and there... make them shot in arm, torso, face too???
That's all I have for now. Gonna go play some more, Maybe I'll think of some more.
Edit : Removed one that apparently already exists. Thanks Chthon for pointing it out to me.