He's one to follow.
Assuming Rakitic and Vidal will be gone in 2020, with Busquets on the verge of being phased out, either de Jong or Arthur can fill in permanently at CDM and we could look at signing a midfielder that can play a role forward.
Havertz is a proper midfielder even if he's more attacking than others so I wouldn't worry about him turning into another Coutinho. He's physically a force, creative, scores a lot of goals.
He need to have another great season though and I would like Bosz to play him as a RCM in 4-3-3 too.
I rate Puig, but I'm not sure if he'll ever be a starter, has some serious physical limitations, nonetheless I rate him highly and he definitely has a significant role to play at Barcelona.
Alena I'm less sure about. In many ways similar to Rafinha I think. Can potentially play multiple roles, but there's just something lacking IMO.