This would be my team for the Champions League.
I know some people here would prefer Roberto because he adds more in possession, but teams in the CL will punish Barcelona severely if both FBs push up high and one of the flanks is basically useless. Therefore I would prefer a more balanced approach with one WB and one FB.
Messi naturally goes towards the middle so I would use Dembele on the RW where he's probably at his best and Suarez start out slightly to the left and then to his CF position. He goes a bit wider to support Alba when he needs it. This way you won't have the right side of the field being empty and useless or fragile defensively in the case Roberto plays and pushes too high up.
You need someone to then cover up for Alba bombing forward so being realistic it'll prob be Rakitic, but Vidal due to his mobility and energy should be a legit option in away games IMO. We're not going to win playing this style of possession-based football as those teams will press us and Busquets and Rakitic being unable to cope with it could bite us in the ass.
Nothing you can do about the CB pairing although I would prefer Umtiti due to his pace. He's simply faster and better than Lenglet so hopefully he can recover to be there from the QFs. Pique is going to start regardless of how he does so again being realistic, nothing you can do about that.
Semedo has not been very good as a RB for Barcelona, but he can play a role similar to what Abidal did. Be a more defensive RB with his great recovery pace to provide width when needed and act as a 3rd CB as cover on counters.
I might try to convey my thoughts about this lineup in a longer post tomorrow before the Super Bowl, nothing BBZ style, but simply more detailed on why and my thought process behind it. Or I might not and this could be my last here one, we'll see.
Anyway, here's the lineup: