Yup, Ancelotti has thought about how he places the position of players. Maybe Ancelotti can put Mbappe as a striker and Vinicius as a winger then push Belingham back. It seems that there are many options that can be made by Ancelotti, in addition to Modric's expiring contract can be filled. There is nothing to lose with the arrival of Mbappe, Madrid will have many player options. That could support Madrid's achievements better.
Believe or not, Carlo Ancelotti is very smart tactician and he cans play well some players not on their real position, Rodrygo had impressive well last several season with Real Madrid and Carlo Ancelotti most of them put him as false nine for central forward although his real position as left or right winger side. I don't think not problem for Carlo Ancelotti if can reach agreement deal with Kylian Mbappe and he will put him as central forward and Vinicius Jr keep at his real position as left winger.
Luka Modric future have been released and Madrid not extend his contract so far, he will be free agent on the next season and his future can't speculated yet because that player not announce yet which one team become his challenge after leaving Real Madrid. Currently, Luka Modric performance not really bad and give him one year contract and he will back up as midfielder position.
Meanwhile according to what I have seen so far, Carlo Ancelotti is a pragmatic coach, doesn't change his strategy too much and focuses more on traditional tactical schemes. Carlo Ancelotti will be more effective if his squad is filled with star players as they have been in the past, the results are very clear. Meanwhile, in the current era, Carlo Ancelotti needs to adapt considering that several of his opposing clubs have developed modern, attractive systems and tactics and also prioritize equality of player abilities in each line. Therefore, when they faced Manchester City, Ancelotti's troops experienced difficulties. Furthermore, regarding Mbappe, I will not speculate too much until he actually occupies his position later. Carlo Ancelotti will also sacrifice one position if he wants Mbappe to be comfortable in the sector he is good at, and here Vinicius will certainly change position.