There's also the possibility that Rick, facing his final days on the show, will just straight up hallucinate Shane like he did with Lori during Season 3. Having kept Negan alive in Carl's name, it would be easy to rationalize the hallucinations as Rick's brain wondering why he spared someone who effectively got his son and several of his friends killed, but not a man he arguably knew longer and could've tried harder to reason with.
Shane was unhinged, Rick would argue, but Shane also didn't beat two people to a pulp in front of their respective lovers with a spiked baseball bat, or pummel a guy to death with his bare hands. These are things that Shane would taunt Rick for, especially a Shane hallucination that comes not long after Carl has died and Shane in real life had to step up as a father when Rick was thought to be dead.
But what if Rick isn't the one experiencing the hallucinations or dreams? There's a chance, even if it's a flimsy one, that Darryl Dixon could have them as well. Since Norman Reedus is set to take over Lincoln's spot as the leading man, and there may need a case to be made for how his idea of leadership differs from what Rick did. Highlighting the different ways each men have or will lead the Group through hardships -- Shane as the past, Rick the present, and Darryl the future -- would be a reflection of how far the show has come, considering that it very well may not get the chance to do so if there's not a tenth season.
We've never really gotten a chance to see Darryl and Shane interact with each other just by themselves, despite both of them being major parts of the show's early seasons. This would be as good a time as any to do so, if only because Darryl will inevitably be compared to Rick and Shane at some point.
Shane is a big part of the history of The Walking Dead; his death at the hands of the Grimes men have arguably set both Rick and Carl on the paths we've come to find them at. Bernthal was always going to have to return eventually, and the show just happened to pick the right time for it to occur. With the show saying goodbye to the Grimes chapter of its history, it's only fitting that Shane come back to bring everything full circle.