I have a small site, 18 pages, with Adsense on it. It's not MFA. It's original content I was sharing online for free on a Geocities site. Then when I found out about Adsense I bought a domain name & redesigned it & started making a little money. Lets say it's about blue widgets. I added one page to the site about something different, lets say orange floogles. Now ALL the ads on ALL the pages are different, they are sort of about widgets and flooges and such, & I guess that's OK. Next I want to put up pages about green wombats, about 10 pages will cover the new topic. After that's done, I can also add pages about red widbats. That topic can be about 6 to 10 pages. All 4 topics are sort of related, subtopics in a very wide main topic. If I add the 2 new topics to my existing site, I can see it will probably affect ALL the ads on all the pages. That could be good or bad, I don't know really. Should I just add the new pages to my existing site, or should I buy a new domain name for the wombats and widbats? The domain name for the blue widgets is specific to blue widgets, so probably I should get 2 more domain names. I guess I should have picked a more generic domain name to start with, but I just didn't realize the potential back then. What does everyone else do? Do you set up a separate website for every new idea?
If there is some sort of relationship between your ideas then I'd put them all on the same site. I've found that the ads do vary page by page and the overall theme of the site doesn't impact much. Compare three fun pages on my site (rather than dropping links to pages I really care about) Beer Winzip Word Find Puzzle The ads are all really different...
Another benefit of keeping everything on the same domain is that Google and other SE's are already coming by daily, and they'll pick up your new pages (for your new products) relatively fast. I.e. you won't have to labor to get into the SE all over again. If you want to separate them, why not just make a folder and drop them in there, and link them to each other throughout the domain.
I went through the same thought process you did. And did both. Expand your current site. But also create new sites. The content do not have to be exclusive to one site.
Another possibility to consider is creating subdomains. If your current domain is wombat.com, you can make blue.wombat.com and pink.wombat.com.
personally I like to set up more different sites on different domains...and then interlink them to each other