I don't know what he will do, but I would suggest not issuing a motu proprio. A motu proprio means that it is, in some way, his personal initiative. In my opinion, it would be better to issue a more solemn document like an Apostolic Constitution and to really regularize and establish a completely new ordinance regarding the matter of the Traditional Latin Mass.
Perhaps not directly retracting or rejecting Traditionis Custodes and Desiderio Desideravi, but instead, de facto, going beyond them by simply declaring that this rite, because of its venerable age, its uninterrupted use, and its deep theological and liturgical roots, must be respected. He could use other arguments as well.
Every Catholic priest and every Catholic should have the right to celebrate or assist at this Mass and to pass on this rite. No bishop has the authority to prohibit or to limit in any way this form of the rite, because it belongs to the entire Roman Church not merely to a local bishop. He (a bishop) has no jurisdiction over this liturgy, I mean no right over this rite.