There are three possible ways for a new Sui Iuris church to come into being…
- An Orthodox Church of a non-western Rite comes into corporate union as a synodal action.
- an extant Sui Iuris Church has become functionally two autonomous churches, and Rome simply acknowledges the factual separation.
- Rome decides to create a Sui Iuris church to accommodate some particular new tradition that has been evolving already within the church.
1 is unlikely to happen due to the Balamand agreement.
2 is possible; some expect this to happen with the Ruthenian church, while others expect instead a second metropolitan and one of the two becoming the major archbishop.
3 is unlikely, since Rome has not granted sui iuris status to any western rite, ever, save the Roman Church as a whole.
The only reasonable candidates at present would be among the Byzantines… there are several that are nominally part of a different church sui iuris, but are functionally separate, but not at present self-governing… they are Exarchates under Rome’s omophor. Rome could make them sui iuris eparchial churches, or combine several into a metropolitan church, or leave them as is.
There is an outside chance of some western Sui Iuris churches, should a Pontiff decide it solves some issue in the church; many were hoping for this with the Anglicans, others for those strongly attracted to the Extraordinary Form (especially since it’s theology is starting to diverge from the mainstream Roman).
Theoretically, a corporate reunion of some body of Lutherans might be able to come into union as a sui iuris body, but it’s more likely to get a system of Ordinariates, just like the Anglicans, and for the same reasons.
But all of this is really unlikely.
If anything, Rome’s likely to combine a few extant ones.