Many users miss the days of desktop pets and animated assistants. A modern take could use procedural animation and simple AI to create characters that react to user activity.
Build a cross-platform app with various creature designs that respond to mouse movements, idle time, and system events. Monetize through premium character packs and customization options.
Start with a basic animated blob that responds to mouse movements, then add personality traits. The risk is novelty wearing off quickly without enough depth.