Stones is the obvious long term successor, given his age and indeed nationality. No rush for it to happen though, we should let him concentrate on his own game for now.
De facto, KDB already acts a captain. If you watch him during the game, he's with every single player telling them where to go and conveying Pep's directions. His centrality to the team is unrivaled and it helps he has multiple languages at his disposal.
Any of Stones, Walker, KdB.
Perhaps let Stones develop a bit more first, but long term, if all goes well, he could be awesome. Walker has surprised me, clearly showing leadership as well as a clean pair of heels. What riches we have now !