Many teams are yet to sign the players they want in order to prepare for the next season. Some clubs are waiting for the World Cup to come to an end before signing more players to strengthen their squad for the Champions League. Moreover, it’s not the same squad we saw in a team last season that we are going to see in the Champions League next season, some teams like Real Madrid and Barcelona are busy signing top players to go far the tournament.
Of course, predicting the Champions League will be difficult right now, the Champions will try to defend the title next season, but we don’t know how other teams will perform in the competition. PSG hardly Mae it to the final, but last two seasons was a bit different, they reached two finals in a row and won the title. Other teams will try their best to stop them from winning the title again next season.
Unless you are waiting to see "how" they will do at the world cup, I do not think that there is a need to wait. So many players that are at the world cup right now, ended up getting transferred already, and that's not a big deal for them. Look at Cucurella for example, dude got transferred a month ago at least, and yet he played fine there. So waiting makes sense only if you are buying someone you want to see at world cup first, if they play good, you get them, if they play bad, you do not get them. But if you already know the player, then there is no need to wait.