A player like Messi got enough respect in any team he played and he usually gets enough support from both players and the fans after Messi left Barcelona for PSG he couldn't get enough respect and he can't have enough focus on the game, that's why I think he is going to leave PSG to another team at the end of this season.
Messi to Barcelona could be possible, it is going to be tough and I think maybe not even smart, but it would be possible. How? They need to sell some players and they need to not resign some players as well. This gives them two things, first of all it gives them money to use for getting Messi and paying his salary when they sell some players, and when they do not resign some of them that opens up cap space as well. Is it doable? 100%. Is it smart to do? I am not entirely sure about that.
Yes he is Messi and he is the legend of Barcelona, probably the best player that ever played, but would you really have 4-5 good players gone, just for Messi? If he was 30 years old I would understand, but he is getting older as well, he won't be able to get them wins like that anymore.