Multi-vendor marketplaces, like ThemeForest, can be very successful. Chose a niche and create a vendor website for it. Your marketplace could be anything, from a platform for local artists to sell their work on, to an online digital product store. Once set up, invite people in that industry to sell their products on your site. You take a percentage of their profits when items sell.
My name is Jamie Spencer and I have spent the past 5 years building money making blogs. After growing tired of the 9-5, commuting and never seeing my family I decided that I wanted to make some changes and launched my first blog. Since then I have launched lots of successful niche blogs and after selling my survivalist blog I decided to teach other people how to do the same.
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.