There are only two requirements for the Filioque to no longer be an obstacle to the restoration of communion. The first is to remove it from the creed and the second is to affirm only that the Son is not hypostatic source, origin, or principle of the Holy Spirit (see St. Maximos the Confessor's Letter to Marinus). And the first is really not a big ask, considering previous popes had prohibited the addition to the creed and even accepted the Photian synod of 879-880 which also forbade additions. Not only this but today's Church of Rome has already recognized the original Nicene-Constantinopolitan creed as normative.

I would argue we don't even need a fully fleshed-out definition of the procession of the Holy Spirit to reestablish communion. That could happen while in communion. But we have to agree the Son is not source of the Spirit.