Do what with the B team? What are the objectives he'd need to meet? Barca B is a developmental team, first and foremost, many better places to prove his worth than in Segunda B. On top of it, the club seems content with Pimienta who does well with the squad and has the right objectives in mind.
Would be perfect for Xavi to coach somewhere in Spain, imo.
Xavi is probably too "proud" to go to a midtable La Liga team. OTOH there are also no clear jobs waiting for him right now.
I think Villarreal would be the best fit for him but don't know if they're ready to ditch Emery for an inexperienced Xavi. Lopetegui is doing great for Sevilla so he's not going anywhere, the same for Pellegrini and Betis or Sociedad who have no reason to replace Alguacil but if they will they'll surely give a chance to Xabi Alonso (who is coaching their B team and knows the club inside out) way before Xavi.
Valencia is also far from a great destination for any coach right now but if he'd go there and do well then I'd believe he's ready for Barca.
And I don't see Xavi at a club like Getafe who could part their ways with Bordalas this summer. He could potentially do well with likes of Alena, Cucu and Kubo (if they'll stay there) but the rest of their team just doesn't fit into Xavi's football philosophy so they'd need to change too much to get him and they don't have resources for that.
Celta could be an option too although Coudet is doing solid and they don't really have good past experience with "Barca DNA" coaches. Sure, Lucho did well in his only season in Vigo before coming to Barca but he's probably the furthest away from a true Barca DNA coach. OTOH Eusebio didn't do to well when he led Sociedad still in Segunda, and Unzue and Oscar Garcia also weren't good for them in La Liga.
So it's easy to say he should get La Liga experience but much harder to find the right club who would believe in Xavi as a coach enough to give him a chance knowing he'll most likely be there just for a season or two at most before he'll inevitable go to Barca. Unless he'd flop there hardly which would make Barca reconsider hiring him but obviously the club that would employ him wouldn't really want to see that.