Plenty of my ideas right now are card games and RPGs, so I'll stick to one of each for now.
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Single Player Card Game: Cats versus Plants - You play cat cards. You are trying to knock over the opponents plant cards. You may play 1 cat per turn. Each cat gets to swipe at the plants. Flip a coin or roll a die to determine if the plant is knocked over, broken, or untouched. Once all plants are knocked down, or destroyed, you win.
Single Player RPG: Band Battler - Psuedo-Combat - You walk into venues. When you do, you encounter another band (Reduced Scope: You are a solo act). You get into a battle of the bands. Instead of reducing hit points, your goal is to move the crowd meter to your side, starting in the middle at 0. Different plinks and plonks effect the crowd and your opponent in different ways. You gain experience and confidence and move on to the next venue.
Single Player Weird Puzzle Game - Could be Horror if you wanted it to be: Please Fix My Teeth - Use weird or normal dentistry tools to fix different issues with this person's mouth. You'll need a different tool for each problem.
Single Player Visual Novel: From the Heart - You play as a character trying to cheer up her friend by writing a poem, and you have to talk to 3 other NPCs to come up with or get inspiration from to write the lines. Could either have a top down 2D traversal, or Psuedo-3D dungeon crawling, or just menus to navigate neighborhood.