You brought some awesome examples. One of the best ways to get your campaign off the ground is by starting with the ideal audience. I find that I can use data to find my top customers and then make those into ambassadors, nurture those relationships, to create an epic viral campaign. Gilles de Clerck, founder of the Growth Revolution, talks about it here - https://sharesomefriends.com/blog/referral-marketing/
Ebooks are one of my favorite sources of passive income. Now, you can do this the simple way and just publish it on Amazon's KDP. Or, you can go all out and build yourself a book funnel. Book funnels are powerful, but they won't be fully passive. For example, if you do a free-plus-shipping offer for your ebook (converting it into a physical book), you'll need to create some one-time offers (i.e. extra training) and up-sells (i.e. an audiobook). But, a book funnel can be very powerful.
At age 55, I own high-end rental properties (near the beach) and commercial buildings servicing the medical industry. I was widely criticized during my career for not living up to my income; that is, buying big homes with many fancy cars. I married a great woman who understood that saving and investing today meant a better lifestyle and more freedom tomorrow. Our passive income is half of my active income from sales, but my net worth has increased substantially. We are both happier and healthier than we were in the high-stress pressure cooker of franchise sales. The naysayers have become converts to the concept of passive income, but they have locked themselves into a “big hat, no cattle” lifestyle. It has been a great ride!
The reason to make use or virality, the ease in spreading and sharing, is however a double-edged sword. We cannot forget that in this type of campaign, a large part of the control falls into the hands of the users, and we risk the message being misinterpreted or parodied. On the other hand, a successful viral campaign can work miracles for your brand’s results.