I've said before I'm also not totally sold on spending 100m+ on Lautaro. Sure he's a very good player and I would gladly take him if we wouldn't already have Griezmann but now I'm not sure. Also, I don't see Suarez leaving this summer and he'll still start most games next season so don't know how to get all of Messi, Suarez, Lautaro and Griezmann in the lineup at the same time. The only way I would go all out on Lautaro is if Suarez will leave.
With Lautaro replacing Suarez we could also play 3-5-2 in some games (but for that to happen we would need at least two new good CBs though one of them could be Todibo). I'm not really a fan of this system but think something like this could work.
---------MATS-------
--Todibo-CB*-Lenglet--
Semedo-------------Alba
---FDJ---CM**/Arthur----
--------Messi-----------
----Lautaro---Griezmann
* fast, physical CB like Upamecano
** a good ball-wining CM who is good on the ball and can also go forward. Arthur can play when we would need more control.
Defensively Griezmann and Lautaro would both track back with Messi staying forward and offensively they are all versatile so they could interchange positions a lot.
Messi is still our RW on paper but he always moves centrally during the games so as long as he's here we won't get a balanced 4-3-3 unless we start to use him as a false 9 again.
But I would still prefer to get someone like Werner or Osimhen and save some money for a CB and a midfielder (those positions are both a priority over a new striker for me) or not bring an expensive #9 this summer at all and go all out on Haaland in 2021 when his release clause is apparently just 75m. I'm sure Bartomeu will want to bring (another) big name forward in a year before elections though so it's either Lautaro or Neymar imo. And between the two I would always choose Lautaro because he's younger, cheaper and (after what happened recently not the least important) bringing back Neymar would further increase the power of veteran players in the locker room and longterm set our required rebuild another few years back.
Forget about Mane, he and other L'pool players are unavailable for a few more seasons because of that Coutinho clause. Anyway, I'm really not sure we need a new LW all that much with Fati here so I wouldn't spend big money on that position. Sure, Fati is not ready to be a full time starter yet but bringing back Neymar would send the wrong message to him imo. Get a versatile RW/LW/CF (Trincao could be meant for this role; I think he'll surprise a lot of people if given a fair chance) or a cheaper (veteran) player who's ready to start big games and give Fati a lot of minutes in "easier" games until he proves he's ready for more.
Manaj is not good enough for the first team. Sure, he could play a few games until the end of the season because we lack in forwards department (but the same is true for Barça B now that Hiroki is also out for the season so I think he'll stay there) but if we'll get a new striker he won't play for the first team this season. He'll probably stay at Barça B for 18 months and then be sold.
This doesn't worry me much at all. Vidal was even more wild and he has settled down since coming here.