There are still a bunch of characters we have not seen. Black Knight, X Men, Alpha Flight, Fantastic Four, Hercules, Spider Woman, Namor, and many, many others. Some were part of the Secret Wars, and I think they need to get them introduced and into the mix before any Secret Wars story can happen.
And most of the villains of Secret Wars haven't even been introduced to the MCU yet. Molecule Man, Absorbing Man, Doctor Doom, Doc Ock, Wrecking Crew, etc. I think Ultron and now Kang are the only ones we have seen yet. So I think Multiverse/Madness arc first, and then maybe the Secret Wars after that.