Assuming Luffy has 4 final crew members, which seems likely based on 4 east blue additions and 4 grand line additions; Luffy's final 4 members will be ally's of the ancient kingdom, and Blackbeard will have 4 crew members that were allied against the ancient kingdom.
The requirements for being a SH crewmember are: have a role in the crew, a traumatic past, a dream, travel with the crew a bit, and it helps to inherit a will or have a good match-up with a BB crew-member.
Luffy's crew members from the new world will be:
Jinbe - a road poneglyph was gifted to the fishmen. Crew job of Helmsman. Trauma of fisher tiger being killed. Has the will of fisher tiger. Has dream to free the slaves and bring the fishmen to the surface.
Carrot- a road poneglyph was gifted to the minks. Crew job of Lookout. Trauma of pedro being killed. Has the will of pedro to bring the dawn. Has the dream to travel the world.
Yamato - a road poneglyph was gifted to the oni/wano (onigashima is in wano, and oars is probably the continent puller that made wano). Crew job of scribe (like oden). Trauma of being starved by father. Has the will of oden. Has the dream to fulfill Oden's will and open up the boarders of wano.
Last Guess is up for debate, possibly urouge or a birkan if there is a giant stalk from elbaf that goes to birka in the next arc? but I'd say a giant, and probably loki's son/a giant small enough to fit on the ship, like the one that shanks was friends with. I'd say Big Mom's poneglyph was probably taken from elbaf adding to big mom being their biggest enemy. My guess is that the giant will have the crew job as an apprentice. Trauma and dream tbd. Also Sanjuan Wolf would be a great matchup.
Potential Members of Blackbeard's crew that were potentially against the ancient kingdom. I'd say 4 join of this list:
Caterina Devon - Being the "crescent moon hunter", that is potentially and enemy of the kozuki, which if the theory of Yamato being a crew member is correct, leads to a great match up as Devon kills beautiful women, would hate the kozuki (oden), yamato doesn't even identify as a woman, is the guardian of wano, and their devil fruits are a fire fox and an ice fox.
Avalo Pizarro - being a corrupt king opens up several potential story lines about how his kingdom was against the ancient kingdom.
Aokiji / another marine - The marines are with IMU / the WG and therefore against the ancient kingdom.
Doffy/another fallen CD - The celestial dragons allied against the ancient kingdom, and blackbeard got crew members from impel down once already.
Urouge - There are theories that he is the 10th titanic captain. Also him vs franky would be a great matchup. Birka has the same name as the moon city. There are theories that IMU came from the moon. Potentially the Birkans were with IMU and therefore against the ancient kingdom. This is dicey as there are also theories that the birkans were with the ancient kingdom and gave the ancient kingdom their tech.
Bonney - blackbeard invited her, and her story that she saw of kuma might change her mind of joining the blackbeard pirates. Potentially to bring down the WG? She is probably of a different race as Kuma is a different race. It is possible that her race or royal blood was against the ancient kingdom.