Don't think we'll buy a CB this summer and I'm also not convinced De Ligt is even our top target for CB position. We are linked to a lot of CBs and in the end probably everything will depend on their availability/price and how they'll develop next season.
That said, I wouldn't be surprised if we'd try to do similar deals we did for Arthur and Mina and use our good relationship with clubs like Ajax (though connections between the clubs are not as great as they were 15-20 years ago; De Ligt, De Jong, Kluivert), Lyon (Aouar, Tousart, Ndombele, Mendy, Gouiri), Betis (Fabian Ruiz, I think Francis is worth to keep an eye on too) to get first/buy option or whatever so we'd basically control their destiny before they go to bigger clubs and become more expensive and/or unavailable if they end up in Madrid, PSG, City, Bayern,... who never sell their key players if some of these players will develop into that. And if they won't then they are probably just not good enough and won't be interesting to us in 2-3 years even if they would be available for a good price.
I think for most of the players mentioned above would be the best if they would stay at their clubs for at least another year, play regularly and develop under less pressure before making the next step in their careers. On the other hand we'll already use next season as adaptation period for Griezmann, Arthur, Alena and based on their injuries/limited playing time this season we could probably say the same for Dembele, Mina and even Semedo (if he'll become a starter). IF Iniesta leaves and Coutinho will play primarily in his position he'll have to adapt to play there as well. So that's basically 5-7 "new" players.
I know people would like to get rid off all our deadweight and replace them with 3-4 20-and something year-old kids and while I'm not against bringing fresh talent to the club we should also be aware this is real life not FM/FIFA and most new signings (exceptions like Umtiti are very rare) will need 6/12 or maybe more months to adapt and integrate into the team even if they are proven players like Cou or Griezmann and more so if they are younger players with limited top level experience coming from (mostly) inferior clubs/leagues.
So this season is used to integrate Semedo, Dembele and Mina (none of them has really made a full step forward so far for different reasons) alongside more experienced Paulinho and Coutinho. Next season we'll try to do the same with youngsters Arthur and Alena (if we are serious about giving him a chance) and more proven Griezmann. Then in 2019 we can get De Ligt or another CB to groom him as potential Pique replacement (if Mina won't show enough to be considered as one) and another midfielder, possibly also LB and RB (if Semedo won't prove to be a longterm solution next season). Slowly rebuilding/rejuvenating the team while staying competitive with our "old guard" is the way to go IMO as long as we have Messi playing at (or close to) his top level. Nobody knows what will happen after that and we can't expect every signing to turn out gold but hopefully most of our recent and future signings will be closer to MATS/Umtiti than Denis level so we'll keep competing for major trophies in the next 2-3 seasons and then our transition period after Messi could also be easier if we'll succced to make a good team around "new" core players like MATS, Umtiti, Coutinho, Griezmann and hopefully Dembele, Semedo, Arthur, Alena,...
