I think the better option would playing with 2 center backs over 190 cm but a big striker would be great too.Firmino - 5.11, Alvarez - 5.7
Even then choosing players must depend on the needs of the team. Barca is relatively short compared to other top CL sides, and last season struggled against Inter when defending set-pieces. Barca will be increasingly facing athletic sides going forward in the CL. I wasn't referring to a 9 that is merely big with brute force, but someone who's pacey, agile, but also offers decent aerial threat and can help out in defense.
Alvarez‘s price is just too high, no move will happen