I could see Mysterio as the 6th member would love to see him on the big screen. Given the timeline they are trying to set up with this new universe its going to be interesting to see how they go about it.
You got ASM3, S6 Movie, Venom Movie, then ASM4. You will have to introduce the symboite and Venom before the Venom movie as a villain in ASM for Spidey before he goes all anti-hero in his movie. If we are going by what they are saying about the spin-off films being in between the 3 & 4 it would have to be ASM3 to give you black suit spidey, brock introduced, and venom creation.
I am new here so I am sure these scenarios have been thrown out there but with the possibility open for Spider-man to be making appearances in both Venom & S6 movies. They leave open the door to maybe have Venom & Mysterio be your villains in ASM3 or Venom/Doc Ock then do the big S6 team up in the S6 movie instead of in the ASM. Then there is the same situation as Venom do they introduce instead of Venom in ASM3 Ock & Vulture since we haven't seen either yet or wait until the S6 Movie to introduce them.
It seems to like with so many characters yet to be introduced it would have been better if the movie line up went, ASM3, S6, ASM4 then Venom Movie. This could all change but just going by what they said at this point just curious with 4 characters yet to be introduced for these spin-offs how they will weave all this together into a nice little package.

Also curious if they will use the ASM movie name tag on the S6 movie for brand recognition like The Amazing Spider-Man: Sinister 6. When it comes time to market movies like it and Venom.