I've very briefly thought about selling chestnut, hazelnut, and persimmon seedlings that I grow out each winter/spring/summer under grow lights. I also grow many of these same seedlings outdoors in air-pruning boxes and gravel beds that I could sell as bare-root seedlings in the fall. I end up mostly just giving them away to hunters or property owners I know. Selling just a few can cover the cost of pots/media, but selling a bunch doesn't really make it worth my time unless I sell several dozen.
On a more serious note, I have a few different irons in the fire on one of my farms. I have started chestnut and hazelnut orchards and will be starting an elderberry orchard this spring. The idea being to gradually build these into a business to continue in retirement. Right now, I think elderberry has the most promise based on some early work. The pie in the sky other thing that I am looking to break into once I move closer to my property is black walnut syrup production.