Although it is true, Real Madrid is a typical club that rarely brings back former players. But who knows, maybe that has changed now. Because if Real Madrid re-signs Ramos, of course this will ease Real Madrid's finances a little. Because Ramos can be brought in for free, plus his salary is currently not as big as before.
In fact, in the last few seasons, especially last season, Madrid have been indifferent by bringing back several players who have actually experienced playing in Madrid before. Fran Garcia, Brahim and Joselu are 3 players who have already experienced the competitive conditions in Madrid before and Ancelotti pulled them back last season to emphasize that their methods are not like before when trying to make Galacticos with the power of money they have.
But to sign Sergio is difficult. He's 38 years old and Madrid won't take any action in defense even though he's a legend and his experience is still very good but still in this case we must realize that Madrid won't expect anything from Sergio for now.
Looking at his age I think it would be more suitable if Sergio retired at his current age. He's a pretty tough defender but at his age at a level like Madrid it's going to be very difficult so if he doesn't retire then at least he's looking for smaller competition.