Among points which we might expect an Encyclical to expound, there are a number which relate to the causes for which Matrimony was ordained.
(1) It was ordained for the procreation of children, to be brought up in the fear and nurture of the Lord, and to the praise of his holy Name;
(2) It was ordained for a remedy against sin, and to avoid fornication, that such persons as have not the the gift of continency might marry, and keep themselves undefiled members of Christ's body.
(3) It was ordained for the mutual society, help, and comfort, that the one ought to have of the other, both in prosperity and adversity.
(4) It was ordained to join together a man and a woman.
(5) It was ordained to last as long as they both shall live.
These causes are incapable of being varied by Popes, Ecumenical Councils, Synods, Cardinals, Germans, Bishops, priests, Judges, Presidents, or Legislatures, because they are inscribed within the very nature of Man. This needs to be made explicit.
The teachings de Usu Matrimonii of Pius XI and B Paul VI should be repeated, together with appropriate sections from Veritatis Splendor para 80 (which usefully includes Gaudium et Spes para 27).
In order to clarify the finality of the judgements given, it would be edifying if the Encyclical cited the Decree of Vatican I Pastor aeternus (particularly the words Faith and Morals); and included sentences with the verb definimus; and summarised its teaching in pithy little sentences beginning Si quis dixerit or Si quis negaverit, and concluding Anathema sit.
To comfort those to whom there seemed here to be new burdens, it could conclude by defining that the Mother of God is Mediatrix of All Graces (perhaps, in order to mark the ecumenical significance of the proceedings, this last could be done in Greek using the words of the great Hesychast Doctor S Gregory Palamas).
There. I've done all the work myself, really, haven't I? The whole thing could be solemnly promulgated next Easter Sunday, with the entire world-wide episcopate present to affirm (Placetne vobis, Fratres Venerabiles? PLACET! PLACET!! PLACET!!!), sphragizein, and subscribe it. Our beloved Holy Father would go down in the annals of Papal History as one of a very small handful of the most doctrinally significant Roman Pontiffs.