Mbappe, Kessie, Brozovic, Modric, Romagnoli, Dembele, Rudiger, Belotti, Perisic, Onana, Christensen, Dybala, Tolisso.....
All those players will be out of contract this summer. Why we never try to get some of those players? We signed only Zlatan for free and that was because Mourinho insisted on him and used his relationship with Zlatan.
I know that they are not completely "free" and you need to give some cash to player and agent but some of those players are in best football age and are worth a lot on transfer market. On top of that, this year some of those free agents are exactly what we need (Rudiger, Kessie or Brozovic).
Last few years, other top clubs signed some very useful players in this way. Real signed Alaba, Barca just signed Kessie, PSG signed Donnarumma and Ramos, Chelsea signed Thiago, Juve signed Rabiot....