Toni Kroos and Luka Modric are expected to be mentors to young players next season because after all the two players have more playing hours and have faced a much higher pressure than the players who are left now. Their presence at the club can actually provide motivation for young players because they both have so much experience in facing important matches and may be true as you say for this reason Real Madrid extended a one -year contract for Luka Modric.
Jude Bellingham will be a new force in midfield and I also want to wait for the latest news about the striker they will contract next season, because relying on the squad they have at this time will make the front line a little weak for Real Madrid. I hope Real Madrid will make a quick choice for next season's striker and they must have quality strikers to compete with other clubs.
Kroos and Modric indeed have a huge experience and they would make sure the young stars of the team benefit from that experience. However it will be only for next season most probably. Because Modric and Kroos could have even left the team this summer but they decided to go for one last season. After that I won't be expecting any of them to continue here. We might be most likely to see both of them outside Europe playing for an MLS or Saudi Arabian team.
Bellingham is the best move made by Real Madrid this summer for sure. Maybe he is playing as a central midfielder but I don't have any doubt that he will support the attacks quite often and play a big role in scoring goals.