I honestly think the best thing for Barkley would be a move to a smaller team looking forward, so he can be what Ancelotti calls "The Technical leader", somewhere he will be the player they look to for a spark, and somewhere he can build some authority and responsibility as a member of a team, without the pressure of playing for the team he supported, or a team that sees anything outside the top 4 as a failure.
As pointed out by others, he definitely has all the physical attributes, but the mental side of his game is damaged. He couldn't handle being the next big thing for us, and he certainly didn't have the mentality to be the next Rooney we all hoped for.
He flourished under Martinez as he took a lot of the fear and responsibility away from him by surrounding him with other young, confident, energetic, players, like Lukaku, Stones and Deulofeu, and hard working players like Mccarthy, Barry and Naismith, all of whom either covered for his shortfalls tactically or gave us another outlet.
At Chelsea, there are lots of other talented players that make it harder for him to stand out, and while there's the added pressure of playing for a team constantly challenging at the top of the table, the added competition and bigger egos of the players he has around him, gives him the scope to fade into the background.
The only way I see him improving at this point, is to take a step backwards, and become the main man for somebody new. He needs to be a Shelvey for a lesser team, be the big fish in a small pond and gain that responsibility of being a match winner, someone to rely on. And if he does that, he might eventually find himself back at a bigger club.
He needs to do it soon as well, because the longer he leaves it, the harder it will be to do. And his next move certainly shouldn't be back to us. I think that would be a disaster for both of us.