It started with Rick, it'll end with him. I think by the time the final season arrives Rick's metal state will have become so corrupted by the brutal world that the line between friend and foe will blur and become indistinguishable. The change will be gradual for believability but I think it will happen over the course of time. Sure, he may seem like back on the path of Sheriff now but we're going to see Rick as a character shift more and more into an antagonist me thinks.
I think we're going to see some brutal and unnecessary stuff from him as time goes on even moreso then now. The brutality of the world in combination with meeting people like The Governor, Negan and other vicious human beings will slowly change his ideals, methods to ultimately transform him into [from the groups eyes] a threat, a threat to everything he and the group have built up and achieved. I truly believe this is how Rick's story arc will go in the long-run. At the end of the series it'll be clear that while he managed to beat all his enemies, create and maintain community and garner the respect of many... the world had beat him.
The final antagonist will be the last nail in the coffin, the trigger for this transformation for Rick to go all the way... this ruthless antagonist will push him to this state. Rick will kill the antagonist in the end but not just at a sacrifice but many unnecessary sacrifices. He's going to do something which he deems justifiable because it got the job done that the group deem as unforgivable. There won't be no 'cliche' redemption here.
This is where the rest of the group will come into play but primarily his son, Carl. Throughout the series in both the show and comic we've seen him make questionable decisions and you would think because of those decisions and how his arc as progressed that he'd be the one to become the "threat" - but I actually think he's stronger than Rick in some ways. He will take matters into his own hand, shoulder the burden himself and kill his Father. While Rick gave into the world, Carl won't. "You will beat this world" - Lori Grimes
We see Rick's death from his POV, it pans on Carl picking up the Colt Python and shooting him in the head. Rick smiles at the end, knowing that Carl did the right thing and that he can finally be at peace.
Flash Forward 15 years
Alexandria or whatever community they now call home will be huge. Great walls spanning a sizeable distance, serving as protection for a couple thousand people. Big expansions, many houses, electricity, defence force and other mounted defences. This won't be Alexandria 2.0, a town trying to rekindle what it was like before - this will be a community that has fully embraced what a post-apocalyptic community needs to be.
The community will have an improved council, built by Michonne. She'll oversee a lot of what goes through Alexandria and forwards any potential issues to the council where they can discuss the matter at hand. Michonne at this point is highly respected, as is Sasha, Eugene, and others that are also on the council. They would have introduced many rules such as everyone must sleep with one hand being handcuffed to help defend from unsuspecting death/walker reanimations.
Eugene beyond the council will be the man behind long-communication with allied communities. He'll have his own 'business' and handle the most important technology advancements within the community.
Sasha will be the commander of the defence force, alongside Aaron and Enid. They'll be responsible for defending the community, improving relations with near-by settlements. Securing trade-routes and their outposts.
Finally, Carl's role is outside the walls. While he was offered a spot on the council, he turns it down and instead is focused on improving relations on a much larger scale. Showing other communities that they can do the same. People will know of him, in a John Connor esque way.