I would scan the auctions to see what's selling well. Other than that - how about gardening supplies - herbs, produce, seeds, guano, hoes etc, etc.
Furniture making supplies
Food making supplies
Drink making supplies
etc. etc. just a few ideas for ya! Good luck!
+1 Diaveborn, right now mostly what's selling is building supplies to make towers, but eventually that will level off, so its best to target what you enjoy doing the most that way it's fun and not work.
If you're into cooking, a restaurant that specialized in a particular type of food would be fun. I've seen a lot of diners, but I haven't stumbled across any Italian kitchens or a Chinese takeout places. The food items exist already, and the decor wouldn't be too hard to pull off given the furniture available.
Like the others have said, it's best to sell things you actually enjoy making.
I'm in the process of making my tower into an apothecary, because I really like growing herbs, and making tinctures and potions.
If you like cooking, sell food.
If you collect cubimals, sell your spares!
If you spend your time harvesting from trees, there's always a market for basic resources like that.
Well, I decided to sell 'Flour' because the name 'Flour Tower' appealed to me. It was going to be 'Flower Tower', but there are only a few herbs that resemble flowers, while there is soo much you can do with flour...
I've opened a Rook museum and gift shop. Slowly building a collection of rook-related artifacts, and stocking rookswort, etc, in the gift shop. Is there any part of the game that especially appeals to you? Pigs, butterflies, herbs, mining, etc? You could create a tower to reflect those interests...