We had a similar topic about the OF. What would you change in the EF if you could? Most of my changes are similar to the 1965 missal.
Here’s what I would change:
Add the universal prayers, but they’d be conducted by the priest, follow some set formulas and be completely optional.
Eliminate the need of the priest to read the parts sung by the choir.
The introit would be sung before the asperges, and adding ancient tropes to the Kyrie would be allowed, in a similar way to the Penitential Rite C of the OF.
Add a reading, like the OF, and I’d make a new lectionary, with a new reading for each ferial day, the readings of the other days remaining intact.
The responsorial psalm and the acclamation of the Gospel could be an alternative to the gradual and alleluia, as long as the psalm be the same of the gradual and the verse of the acclamation of the gospel be the sabe as the one from the alleluia.
Write a new, shorter confiteor, but preserving all the main elements of the EF confiteor.
Allow some traditionally silent parts to be said aloud: the secret, the consecration, the anamnesis, the doxology of the Canon, all the prayers from the end of the Pater Noster until the Agnus Dei and the last gospel at sung masses.
Allow missa cantata with one acolyte.
Allow singing the ordinary at spoken masses; the propers could also be sung, but only in psalm tone.
Allow the Pater Noster to be chanted by everyone.
Allow the final blessing to be chanted, the prayers before the distribution of the Eucharist too.
Add longer, solemn blessings, such as the ones from the OF.
Allow bishops to celebrate priest mass, or at least make some adaptations to allow high masses with less assistants and make an adapted missa cantata (not high mass).
Readings executed turned to the people
Permit vernacular in the following situations: readings (including the last gospel), the prayers of the priest from the propers (collect, secret and post communion), the responsorial psalm and acclamation of the gospel with the bishop’s consent, and, in dioceses with a great number of illiteracy, all the audible parts of the mass, except for the consecration.
Move the dismissal to after the blessing.
Obligatory leonine prayers in all Sundays and first class feasts.
I think that’s all