I had an idea about a mod that lets you leave notes for NPCs. (The most successful chance of them finding the note being their house or place where they hang around a lot.)
This came up to me when I robbed Orgnar, the innkeeper in Riverwood. I wasn't seen, but the bastard figured it out and sent hired thugs after me later. I didn't wanna kill him (yet), but it sucked that I couldn't intimidate him or even confront him about it (nor did he give a s***). I imagined how awesome it would be if I was able to leave him a sinister and threatening note that would keep him on his toes.
You could leave many kinds of notes: friendly, romantic, threatening, ask to meet somewhere... those are just some from the top of my head. I'm guessing you'd have to leave the NPC for a while before they read the note and then when you come back, their reaction to you will be different: angry, frightened, "It's a fine day with you around ".
The kind of mechanic I had in mind was that a letterbox pops up "What kind of a note do you wish to write?" and you select what kind and then it lets you freely write (type) on the paper whatever you want and then you drop it in a certain place. Would be cool if NPCs could reply to you too.
And you'd need to collect a roll of paper and charcoal.
Now, for the second idea: Better Secured Valuables.
I know this feature is somewhat featured in Skyrim Radioactive, but not fully. There are still coin bags placed right there on the counter for you to sweep when the owner isn't looking.
Don't you love it when you walk into your inn and see a perfectly placed coin bag along with coins just lying there on the table? You just have to close the door behind you before nosy Delphine walks in and steal it as fast as you can and voila. Inn robbed.
Who the hell leaves all their savings lying around on a table IN A GUEST ROOM?
With this mod, NPCs would actually protect their valuables, store them in a safe or just hide them from plain sight (better than just jamming coin bags between a bed and a cupboard). I know some of them already stored some jewelry in a display box, which is alright (even though a non-display ordinary-looking box would be more secure), but some of them are just plain sitting ducks for thieves.
(Sorry if these mods or something similar already exists, I didn't find any.)