I don't think Real Madrid, Manchester City and Bayern Munich will be making big transfers in January which could really have a big impact on their title chances.
I have mentioned these 3 teams because these teams are the top favourites for the championship in the Champions League (according to bookmakers I mean). I also agree with this classification honestly because they are technically the strongest teams with the best squads. I don't think that any of them will feel any weakness in their squads much. Real Madrid could make a transfer for the defense line though. Because Alaba has also got injured so Ancelotti needs to strengthen the position.
These team you mentioned apart from Bayern Munich, they have actually made it official about their intentions to make signing by January but whats not clear at the moment is which players they are willing and ready to sign at the moment, they are yet to have an official deal with any of the players of their choice but they are sure making signings by the end of the season.
Real Madrid for sure needs a Player to help them especially with their striking, if they will be sure to win the UCL then they definitely need to get a player who will help them get that position fixed if not it will really tell on their performance and their numbers of goals will not be as much as maybe expected of them because they lack the force for such. The limitations Bayern Munich will be having regardless of their strength will be experience but then it will be a thing of delight to see the UCL going to Germany by the end of this season.