Van dijk is new signing, he wasn't fit enough when he came but it was clear how the team improved with him at the end of the last season. All we have to do now is find the right partner for him - Lovren can be a good defender on his day, same for Matip but we need a less error prone centre back. Gomez I think he would be very good at CB, he has the physicality and pace and he could simply be one of those players who needs to be constantly playing to find his form. The difference between having another center back close to Van Dijk's level over having to rely on Lovren or even Matip could be the difference between fourth and first.
Wouldn't surprise me if Virgil scored 4-6 goals this season. He missed a few sitters last season against Everton, United and palace but it's a matter of time before he turning every corner into a goal. He's a giant.
Only one season we have conceded less than 40 goals since Rafa's left and that was last season with 38 goals! With Van dijk, Alisson, Fabinho and Keita I'm sure we won't concede more than 25-30 goals.