To be honest, he's not worth £40m and that money would be better spent on Kroos, imo. He's worth about £30m, £35m max and anything more than that and I wouldn't mind if we don't buy him. He's been in no sort of form for a good two-three years now.
fabregas despite how much i hate him at barcelona (not really to his fault, just that his prescence imbalanced the greatest team in the world at the time of his arrival), fabregas is truly one of the top 5-10 centre midfielders in the world with his ability. He's miles ahead of kroos. I think kroos just gets so damn overrated. There's nothing he can do that fabregas can't do better, really. It's different if kroos was well rounded, or physically strong, or had a special outstanding trait, but he doesn't, he's very similar to fabregas, a little stronger and more physical, but nothing impresive or outstanding for the midfielder position, and fabregas is still far far ahead of him in terms of passing ability and vision, technical ability too (touch, ball control).
The fact that barca and fabregas just hasn't worked is simply because of how idiosyncratic being a midfielder for barcelona really is. you have to change who you are and adapt to the rigourous technicalities and details of their possesion play. When over the past two seasons under tito and martino that they've been more free flowing and direct, cesc has been the most effective out of iniesta/xavi/busquets because he still has and always has had one of the best final balls in the world. Barcelona doesn't play in a way complimentary to cesc, and his own play, while effective at times and quantifiably significant in terms of production (assists, goals), has never been complimentary to the barca way either. it still doesn't change or diminish his ability that he still shows on the field. he's world class, through and through.
i don't think fabregas is a real option though, just because of his arsenal history. it would truly feel a little wrong if fabregas came to us, desecrating. It would be like if Ronaldo went to city imo. it's not something you want to see happen and i'd feel uncomfortable cheering for cesc for many monthes if he came to us. as a footballer it's a no brainer to get cesc if you could, but due to his allegiences, i'd rather look elsewhere. I think we need a defensively competent midfielder much more then a playmaker anyways imo.