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.
If you love to travel and find yourself randomly searching for airfare sales or browsing Lonely Planet, why not carve out a niche for yourself as a private travel agent? My friend, Mark Jackson did just that, making extra money online with his travel consulting side business. Start with word of mouth recommendations from friends who know they can count on you for the cheapest flights, and then move on and create a Facebook or LinkedIn group to invite people who want to stay on top of the latest deals. Eventually you could spin this into a full-time consultancy teaching people how to make their dream trip a reality.
There absolutely is still room for new affiliates, because there are always new products to promote and new audiences to reach. The internet is a vast place with infinite verticals to work in, so instead of just focusing on the biggest affiliate programs in a specific industry I recommend using that one as well as others (and Amazon!) to round out your affiliate offers and thus your revenue.
As long as there is still the written word, there will always be editors. Freelance editing and proofreading not only pays a decent hourly wage, it also gives you the chance to read about potentially interesting topics too. What's more, pursuing freelance writing & editing as a business idea can afford you a lifestyle that lets you travel the world as a digital nomad. You can find lots of job postings from companies and individuals in need of writing, proofreading, and editing services on Upwork, which makes this a high-demand opportunity to make money online.
But don't make the mistake of thinking this will be a passive source of income—you're on call whenever you have a guest and you'll always need to keep the place clean for incoming visitors. On top of just renting on Airbnb, consider offering your guests paid add-ons, like Lauren Gheysens', Royal Day Out in London, England—where she gives visitors a local's only tour of the city, complete with bespoke 18th century costumes.
Consider selling in lots. A lot is a collection of similar items that is sold in a group. For example, if you have a collection of books, magazines or similar pieces of jewelry, consider selling them all at once in a lot. You many not make as much money as you would have if you sold each item separately. However, the items will likely sell more quickly in a lot than they would individually.
If you have pop-ups and optins on every page, I would have to think that it would be a lot harder to convince someone to link to it. Especially since the pages where you have your optins (and probably receive the most traffic) are probably the same pages that you have your best content on as well that you'd be hoping to pitch in your outreach campaign.