If Thiago really leaves, Sergi Roberto being promoted is the logical replacement. He also won't complain about having a secondary role.
Sergi Roberto plays the Barca style. He can step right in, this would be a perfect role for him next year. You don't need a world best at every position. It is impossible to keep players for jobs like #4 offensive midfielder who are good enough to aspire for the Spanish national team.
Thiago will not be the last very talented player to choose to leave Barca. It's not a bad thing, they play limited minutes for Barca for a couple or a few years, then they move on. This pattern is what permits Barca to have that type of talent beyond the first 11.
Tito used Song and Busquets in midfield at the same time in some games. I'm sure Adriano could play in midfield. Of course, Messi could, too.