Many great wishful evil suggestions in this thread! I like it!
Here are my "dream" evil scenarios:
0) Meet Maru & Leah
Maru is a nurse by day and a tech/robotics geek by night. Leah, having lived in the big Zuzu City with her ex Kel, can still somewhat remember how some of these "comfangled" high-tech gadgets like cell phones and high-powered computers work, even though she is trying so hard to forget all about that stuff. However, with some gentle persuasion by the farmer, she might end up talking about them and giving the farmer some nefarious ideas! As for Maru, the farmer would need to be very careful when speaking with her to get information on how to "craft" high-tech gadgets. Surely, she would be able to detect if someone has evil intentions in mind!
The "high-tech gadget" that the farmer is trying to learn to craft is an "ultra-secretive bug" that, taking advantage of "micro" technology made possible by Iridium and/or Radioactive Ore, can secretly record every conversation in a room each day and avoid detection -- as long as the farmer is careful to put it in a place that is not accessed very often!
By setting up a computer network at home (Dwarf Gadgets) and checking secret conversations at the end (or start) of every day, the farmer could potentially learn helpful things from their unsuspecting victims such as various secrets to being successful in their businesses or some juicy gossip that could be used to blackmail certain NPCs if the farmer chooses to go that route.
1) The hostile takeover of Pierre's
Are you all sick of Pierre taking ALL the credit for your good produce and *blaming* you for produce that he considers "sub-standard"? Well, perhaps, we could DO SOMEHING about it now! First, by planting a bug, my farmer figures out Pierre's secret to making Vinegar. Next, they figure out how to grow Saplings from Fruit Tree fruit seeds. Once those secrets are learned, the farmer really has no further use for Pierre. My farmer then sells those items at a marked-down price, giving Pierre an eerie sense that his business is in trouble! To put fuel on that fire, my farmer spreads nasty rumors about Pierre using info that was learned from the bug devices. Release of multiple secretly recorded conversations with certain NPCs and his own words to the general public may implicate him and look bad in from of the whole town. Ever-increasing social pressure could end up forcing Pierre to sell his business to the highest bidder -- perhaps the Farmer themselves!
2) Hostile takeover of Marnie's
Are we all sick of Marnie spending all day Monday and Tuesday just blankly staring at her microwave when you really need animals such as Chickens, Goats, and Pigs? Not to fear! With the help of a few secretly planted bugs, the farmer may learn where she gets her "seemingly endless supply" of animals from or how to breed certain animals like Rabbits and thereby make Marnie's shop completely useless to the farmer! Possible blackmail options might include finding out what Marnie REALLY does in front of that microwave all day on certain days. Also, secrets between her FLING with Mayor Lewis could totally come into play if the farmer is interested in taking over her business!
3) Robin's, Willy's
Okay, I admit... those 2 are toughies! First of all, Robin seems to be a real fan favorite amongst the Stardew Community so I'll probably receive LOTS of flack -- for even THINKING of a possiblity that the farmer might one day take over her business. Besides, with the Farmer having learned many things from Maru, Robin's very loving daughter, Maru would be smart enough to connect the farmer to all the possible "nasty rumors" that they might learn from bugs! However, the farmer could still learn a thing of two from Robin such as how to build stuff REALLY fast! Perhaps armed with that new knowledge, the farmer might be able to spend certain days just tirelessly working on building a building and let the sprinklers, kegs, preserve jars, etc. do all their other work for them! Same issue with Willy, being somewhat of a fan favorite. He could theoretically be made "useless" if the farmer learns how to craft everything he can teach, learns all the Fishing Crafting recipes, and buys enough Iridium Rods for every area they can imagine!
Well, that is my "Evil Brainstorm" for vanilla Stardew!
If there is enough interest, I could also write one up for Stardew Expanded!
Have a nice day!