Fabregas's style is completely different from Xavi's and limiting him to a distributing role doesn't get the best out of him
Gundogan is good but Barca don't need to buy him
Verratti is more of a Pirlo/Alonso type player and would only leave PSG to go back to a Serie A club
Roberto is not a specific enough player to be declared "Xavi's replacement". He's all round, box to box, overall very different.
Thiago thrives in that role but he has his own style of play so he wouldn't be a carbon copy of Xavi (nor should he try to be, his style adds something new). He's the ideal replacement, but he has his own game and he is his own type of player so he shouldn't be he shouldn't always be labelled "next Xavi"