Septicemic and pneumonic plague are really just somewhat different manifestations of exactly the same pathogen-
Yersinia pestis. They aren't different organisms, and not even different 'strains'. It's simply that one infection is in the lymphatics, one in the blood, and one in the lungs- and one can turn into another if the organism spreads elsewhere in the body. Pneumonic plague e.g. almost always develops from a bubonic outbreak, and then runs wild. And frankly
pneumonic is the most frightening, as it can be spread directly between people via coughing that produces aerosols, rather than needing fleas. And since you
inhaled it, you get the pneumonic form, too. And fleas still work, too, though you're more likely to get bubonic through that route.
Of course, you could indeed posit a different strain of
Yersinia pestis that
is more likely to go septicemic. That's easily within the realm of possibility. But why? Pneumonic is easier to spread and essentially 100% mortal if untreated. I'd say a strain that is more likely to turn pneumonic would be the ticket, here. While you're at it, make it more virulent (i.e. easily spread between victims).
Moving on:
It's actually pretty easy to envision a pathogen that could kill most of humanity, especially in the modern world with all of the travel going on. It's been described above- highly mortal, but with a
very long and
highly virulent infectious latency period. Fortunately no known organism qualifies- it would almost certainly need to be engineered.
Almost...
From a world-building/book-writing perspective superplague is my favorite way to produce post-apocalyptic settings, though I may be biased because I'm a doctor. To make such a fall-of-civilization setting you need a
truly massive population die-off. Nuclear war has gotten a lot less likely, and a lot of people have utterly unrealistic ideas about it, anyway. EMP and CME are just survivalist fantasies- yes clearly difficulties would ensue but there is no way they lead to unrest and die-offs commensurate with a long-term collapse of world civilization. Asteroid strikes are also low probability, and difficult to get the population die-off
just right, and even then it's likely that part of the world would get totally wiped out with survival increasing with distance from there, so you pretty much have to set the story in one small area that had a few survivors and make the rest of the world dead. Which is also a problem because the world ecology is likely to collapse and kill everyone anyway. We
have gone through worldwide financial collapses and we've still got our cable TV, so that one's a non-starter, too.
So, that leaves superplague. It leaves the rest of the world essentially untouched, barring the occasional containment failure, etc. It would also result in a common post-apocalyptic trope: xenophobic communities who don't like strangers (because in the past wandering strangers were turned away or killed in case they carried the plague). This would also contribute to the loss of central government that these settings require, since during the initial outbreak people are violently resistant to "guvmint men' coming in to re-establish authority for the same reason- they might carry the plague. And for real weirdness make people who get the plague and
survive have characteristic brain lesions that cause fantastical thoughts and/or obsessions, leading to novel cults and belief systems, Immortan Joe and his warboys, worship of the God Elvis, etc., as well as weirdness derived from real-world religions and belief systems such as the CUT in the
Dies The Fire setting.
And of course, zombies work, too.

A Yellowstone eruption might have potential, but again everyone has their own expectations and disagree about just how bad it would be.