Xavi is a unique player, and honestly, when he plays well, he's the only thing keeping this very risky 4-3-3 together. I can't think of any other player who'd fit into his shoes well. Pirlo and Modrić are the closest thing I can think of. Obviously, Pirlo isn't any younger than Xavi, and Modrić is inconsistent nowadays.
Anyways, for me, two obvious choices are missing in the poll. Busquets and Iniesta. Nowadays Busquets and Xavi play on top of each other anyway, and occupy pretty much the same position, because Busquets is not the type of DM who drops deep between the CBs when the team is attacking, but rather even goes a bit forward. I can also see Iniesta dropping deeper as time goes on and evolving into a more distributing type of role. Both are great playmakers.
Having said that, I agree with some of the posts in the thread. You shouldn't search for exact replacements for players who are retiring/leaving, but rather work with what you have and adjust the system to the players, not the other way around. Especially when you have trouble replacing/fitting several players.
For example, a 4-2-3-1 with Busquets as a deep lying playmaker is a possibility. Mascherano or Javi Martinez would be the perfect type of DM to play alongside Busquets in that case. A 4-1-3-2, as I've suggested in the "Barca next season..." thread, is another idea. Anyways, I don't see the need to insist on a system that worked perfectly for one set of players, but probably won't work nearly as good for another, when the squad evolves. I abhor the idea of just buying a bunch of players in order to fit them into a pre-determined system. We are Barca, we have the best youth academy in the world, and many other great players outside of it, and it would be a shame not to use the players we have in the best way possible, because we insist on a system that doesn't fit them.