I have two questions that aren't answerable by reading the graphic novel...
1. So what becomes of the scavenger group? Will we get more back story on this group next season? Or will they simply be eradicated by the combined groups of Alexander-Hillside-Kingdom? Obviously the writers could not give much information away or it would have been even easier to sniff out the "stab-in-the-back" by the group.
2. Oceanside. What happens to them? My guess is they simply show up at one of the allies-camps en-masse.
For the first question I believe they're basically a filler/time killer to buy time for what happens after the attack on Sanctuary.
I believe next season will start with Rick and company taking the fight to Negans house, like in the comic, and I believe it will end where you're not sure if Negan will get out or be over run by walkers.
In the meantime the group will then focus on The Garbage pail kids and destroy them. I'm thinking they'll simply kill them all.
This will serve two purposes. 1. It allows Rick and company to think they've taken out all of their enemies and 2. It's going to set up some big moment, discussion, realization for Rick (probably through Morgan once again) that Rick is now going to far, and really no better than Negan.
At that point Negan will unsurprisingly return and attack Alexandria again, just like in the comic, forcing everyone to flee to Hilltop for the final battle in the war.
Which will lead to the long awaited showdown between Rick and Negan (one on one) and people will be left to wonder rather Rick will in fact kill him, or if he will decide (like in the comic) that he must let him live to prove to everyone that Negans way is not needed in order to survive and thrive.
That's how I think that all ties together next season.
As for #2...I think they're actually going to be left right where they are but set up very nicely as the port community that will be in place after the war is all over and the show goes into the two year time skip.