The season is long and everyone will have to contribute. Lucky for Barcelona, they have great depth in their sqaud and a lot of quality substitutes. Here are the ones who I think will play vital roles in the upcoming season:
Zlatan Ibrahimovic - after a disappointing first season, Ibrahimovic is looking to improve his performances. It is my belief that he might be very useful, even if Villa's arrival will reduce his minutes. Having a world-class substitute always ready to come into the game is a major advantage for Barcelona. Plus, if Ibrahimovic manages to find his best form, he might very well gain a spot in the starting 11.
Bojan Krkić - 2010/2011 might finally be the breakthrough season for this immensely talented player. Just like in previous seasons, he will be given opportunities by Guardiola. If he manages to grab those opportunites, he might very well earn a spot in the lineup eventually.
Maxwell - An alternative for Abidal. Maxwell might not be the best defender in the world, but he's very efficient going forward and might be useful at times.
Adriano - Another new player. Adriano can play both as left back and as a midfielder. He won't be a regular starter (supposedly), but his versitality might come in handy for the team.
Jeffren Suárez - He will probably be given more opportunities this season. The 22-year-old winger is fast and has excellent dribbling and crossing skills. He is the future of Barcelona (just like Bojan) and has every chance to shine this season.
Throughout the season, Guardiola might also promote some youth players, like Thiago Alcantara and Jonathan Dos Santos, two incredibly talented players.