Instead of Neymar, Barcelona should be targeting at least two of the attackers listed below. This is under the idea that Messi would play as a classic number 10, which I think will extend his career into his later 30s, and to surround him with three very active forwards ahead (or two top class strikers) in addition to 2 or 3 secure midfielders depending on the alignment (2 versus 3 center backs).
Nicolas Pepe, Nabil Fekir, Hakim Ziyech, Heung-min Son, Lorenzo Insigne, David Neres
Pepe and Son can lead the line as well as play as wing forwards. Fekir can cover every attacking midfield spot as can Ziyech and Insigne. And if you're determined to go Brazilian, Neres is the one to target. Aside from Son perhaps, all of these players are either on the market or easily acquired.