I'd like to start with Busi - Arthur - Frenkie for the majority of the Camp Nou games in the first half of the season. Let's test it hard for 2-3 months and see if we can build a solid functioning midfield trio out of this formation.
Vidal and Rakitic as first rotational options, especially Vidal. These two would be useful away from home where games are a bit more physical.
Roberto I'm skeptical about, he always had some occasional good game as a midfielder where he made you think 'maybe, just maybe he can be a midfield starter here', but overall he more or less disappoints there.
Arthur and Frenkie starters 2/3 of the season should be mandatory. The 3rd starting spot is wide open, as are the rotational options.