Edgar Rice Burroughs was a professional in the best sense of the word. This meant he worked hard at producing the best work he could. It also meant he knew you didn’t stop a successful franchise but always left a back door for more stories in the future. With Tarzan, Pellucidar and John Carter he used pretty much the same method (which I think was largely instinctual and certainly not planned).
Stage One: The Original Trilogy — write a trilogy (usually the best and freshest of the series) first. The arc that runs through Tarzan of the Apes-The Return of Tarzan-The Beasts of Tarzan and A Princess of Mars-The Gods of Mars-Warlord of Mars feels complete. For David Innes this was only At the Earth’s Core and Pellucidar. ERB could have said I’m never writing about these guys ever again. (Commercially insane and he never said it.)
Stage Two: Bring On the Kids — having finished with Dad, you move onto the son. Son of Tarzan, featuring Korak, or Thuvia, Maid of Mars, with Carthoris (and later Llana of Gathol), means having a new hero, not so much different than the old hero, take up the sword, or vine, or whatever. None of these Children of … ever did as well as their old pa but things go on.
Stage Three: Friend of a Friend — having run out of relatives, Burroughs would pull some random dude onto Mars or into Pellucidar to take up the reins. Tanar of Pellucidar, Ulysses Paxton, Pan Dan Chee or Von Horst, these replacements are stand-ins and things pretty much go as usual…
Stage Four: The Old Guy is Back — When you can’t bring in any more doubles, you simply go back to the heroes that made you famous in the first place. For Tarzan this was much quicker than John Carter or David Innes. When Son of Tarzan bombed ERB swore he’d only write about Tarzan, so he skipped Stage Three (or did he? What about Nu of Niocene?) Tarzan took over again and stayed at the helm for 19 more novels. For John Carter and David Innes it was the Pulps, with Hugo Gernsback or the short stories for Ray A. Palmer that brought them back.
How to use this method: first, you need to create a wonderful novel or two, then follow that up with some movies, and then still more books. And when that doesn’t work anymore, publish them yourself. So is this going to work for most writers? Probably not. ERB was both a master storyteller as well as a guy who lived at just the right time to hit the soft magazines, the Pulps, then the paperbacks. So build a time machine and go back to 1914. (And when you are done, can I borrow it so I can get brand new copies of all those old Pulps….)