We have been linked with alot of promising young defenders lately, who do you guys like thus far examples Dayot Upamecano, Yerry Mina, Davinson Sanchez etc..
Add Santiago Bueno to that list.
I haven't follow anyone of them enough to say who is the most talented or who would be the best for us and they are also at different stages of their development. All of players you mentioned would most likely be signed for first team (with maybe Upamecano willing to go on loan for a season while I think Mina and Sanchez - he already rejected that in summer when we were interested - most likely wouldn't accept that) while I could see Bueno accepting playing for Barça B for a season or two at his age and then we'll see how he'll develop.
So it really depends on what our short time plan is with Marlon, Masche and Mathieu. There are rumours we might loan Marlon next season to get La Liga experience (I believe he's good enough to be our 4th CB but at the same time if we can find him a good team where he'll play every week I'm not against that either) and then we'd need someone who could step up right in as our 3rd/4th CB if Mathieu leaves in summer. That's where I think Mina or Sanchez could be useful, because they've played at higher level than Marlon so far (but on the flip side they'd need to adapt to our system, something that Marlon already had). Then we can return Marlon in 2018 if/when Masche will leave after WC. If we keep Marlon next season as our 4th CB then we could look for someone like Bueno for B team and let him develop there for a year or two. Of course if Masche leaves already this summer then I'd prefer someone with more experience at high level but still young and not too expensive. But as I said I believe Masche will stay here until WC2018.
BTW, I find it a bit strange there are no rumours about Jorge Mere. Not talking about Barça exclusively but I haven't seen any other club being interested in him for months now. I still like him and he's one of the most talented spanish CBs (they don't really have much talent at that position right now though compared to most of other positions) who is playing solid for a bad Gijon team. Just wondering why there is apparently no interest even from clubs like Sevilla, Villarreal or at least midtable EPL teams who are usually throwing money around like crazy for less talented/worse players.