I am hit by a sudden idea: could Kraven be the 6th villain of this version of the Sinister Six?

It is unlikely that the grouping of variants from alternate realities has only five members especially given that Sony had planned a 'Sinister Six' movie for so long.

Green Goblin & Lizard replaced Vulture & Mysterio but, otherwise, Doctor Octopus, Sandman & Electro are founding members of the original team of the comic book.

So, it may be the case of the eventual last member as well?

Jimmy Kimmel was joking but, maybe, he got it right.

It is already confirmed that Aaron Taylor-Johnson will portray the character.

If Spider-Man is chasing "ghosts", people who "died fighting Spider-Man", wouldn't that be the case of the main character of 'Kraven's Last Hunt'?

I may be wrong but I don't think that either Venom or Morbius could be the eventual 6th member.

The post-credits scene of 'Venom: Let There Be Carnage' suggests that Venom would make a cameo appearance but, contrary to the variant appearing in 'Spider-Man 3', this one has not met Spider-Man yet and, thus, holds no grudge against him.

Morbius seems to live in the same reality than the Vulture and, thus, the MCU variant of Spider-Man. So, apparently, he isn't from an alternate reality.

I think Mysterio will be back as the sixth member. He was killed by Spider-man also.

It is an excellent remark.

If the spell retroactively changed the fates of the Vulture & Mysterio, it may have resurrected Mysterio.