Keep a counter on each shopkeeper representing the number of times a pet has picked up a shop-owned item in their shop (or a shop-owned item inside containers they are carrying). Every time this increments, use it as an increasing chance that the shopkeeper will become aggressive towards pets picking up their wares, with messages like “Kabalebo shouts “Hey! Drop that!” at Fluffy. Fluffy drops [item].” Once they are past that point, you can’t use pets to take their items anymore.

The point on the counter at which the shopkeeper stops playing nice should be roughly some fraction of the area of the shop, to strike a balance between large shopkeepers being more lenient and not allowing you to clean out all the tiny shops in Minetown. A rumor could be added for this: “They say that mom-and-pop stores have lower tolerance for thieves.”

Unspecified whether the counter should increment when a non-pet picks up an item in the shop. Probably not, since this is a fairly rare case.

Collect a bunch of Evil Patch Ideas and turn them into scary false rumors.

A much more fleshed out interaction system for sapient monsters. It is initiated by chatting, which opens up a menu offering four key verbs: Ask, Tell, Show, and Give.

Ask is used to gain information from a monster; this could be a whole bunch of things, from rumors to identification to information about the dungeon. Each type of monster you can ask things of has a limited knowledge base, and this knowledge base may contain some inaccurate information. Especially interesting is the possibility that you can ask someone for identification, and they tell you what they think it is - which could be right or wrong, but a good clue either way. You can also ask someone to join you, and pay them some gold to tame them for a certain amount of time.

Tell is currently a less developed part of this idea; the only thing suggested so far is that you can tell a creature about traps you’ve discovered, which will make those traps known to the creature. You can also use “tell calm” on your pets to make them refrain from attacking peacefuls.

Show lets you select any item from your inventory to show to the monster. This can be used for intimidation (e.g. showing a stack of iron skull caps to a goblin).

Give lets you give items to a monster, which in some cases, depending on the AI, lets you test the effects of items.

Note that a potential pitfall of such a system is that if it’s complex enough to do interesting things with, it is probably going to be hard to use without spoilers. And if it’s uncomplex enough, it runs the risk of most possible responses being “I don’t know/care about that”, which also makes it hard to use without spoilers. A middle ground between these is likely to require an enormous amount of work.

Some random graffiti is not rumors; it contains helpful in-game information. Things like “NR 9 is a scroll of earth” (giving object identifications), or “The Big Room is on level 12”.

Add rumors to FIQHack that warn about dragons, which can be fairly dangerous.