I can`t obviously recommend anything to Barcelona, just state my opinion. Dembele and Coutinho sounds good to me, because if Dembele fails as a LW, Coutinho can play that role. With a change of formation we can then maybe try Dembele on the RW. If Dembele shows that he can be great at LW in Barca, Coutinho can play Iniesta role, as he has shown in Liverpool.
I also think that we need a CM, maybe Seri, but I don`t know if he`s good enough to play at this level. I`ve never watched him play, except some youtube clips. And we have too many mediocre players now in the squad, especially midfielders.
If not, better to put the money in bank and wait for an opportunity to snatch someone.