With various non-Catholic denominations it varies.
we are closest to those who acknowledge the True presence of Christ in the Eucharist like the Lutherans and to some/more extent with Anglicans and Methodists. At the other extreme, there are groups with next to nothing in common with orthodoxy -- with those we should be united on common purposes like anti-baby killing etc., and be content to agree to disagree.
I see a good path to agreement in an effort to restore the religious nature of marriage. To separate it from the secular realm and return to appreciating its sanctity.
Other than the definition of marriage in which they all agree, the one thing they must do is to only recognize marriages that fit this definition.
That is, if a couple enter any of those churches, no matter the denomination, and claim to be married, that marriage must be certified as a “real” marriage, by the rules. If it does not conform, then they must be “remarried” by that church, but only if they *can* be married.
This would be the minimum requirements, and denominations could add on to these requirements. The same rules apply to divorce. If it is permitted, it must be done by the church, or it will not be recognized.