Re: Is it possible to world build a franchise that's originally based around 1 hero?
Is it possible? Absolutely! But yeah the spin-off characters and world building started early on and grew exponentially.
Great examples are:
Superman:
-Supergirl, had her own comics, movie and TV series
-Legion of Superheroes, spun out of Superboy, had comics, their own cartoon, and potential spin-off TV (see current Supergirl)
-Lois Lane and Jimmy Olsen both had their own ongoing comics.
-Steel had his own comics, and movie.
-Krypto the Superdog had his own back up stories, and his own cartoon
Batman:
-Batman Family of comics
-Batgirl, comic, TV(Birds of Prey
-Huntress, own comic, with Batgirl spun out into TV Birds of Prey.
-Robin in comics and several animated series (sans Batman)
-Catwoman, comics, movie.
-Nightwing, comics.
-Gotham Girls, comics, animated web-series.
-Mystery of the Batwoman Animated movie
-Jason Todd, comics, animated movie under the Red Hood.
-Damian Wayne, comics, animated movie Son of Batman
-Gotham PD, comic, TV series.
-Harley Quin, comics, cartoons, upcoming film, with several other Bat sirens.
-Spinoff characters with their own comic, to many to list.
For a character who has been solo "for most of the franchise's existence", doesn't seem easy.
If there weren't characters or broader concepts developed to even spinoff, not sure why you'd even want to?
James Bond has a pretty well developed world, although focused on one character, the premise is strong enough, if done well you could suddenly focus on on an all new agent.
Not sure who would want that though? There is plenty of unique secret agent characters out there.