Apart from the arbitrary exclusion of non-Nicene Christians,* this is more or less what I had in mind with the Meta-Church and cross-denominational cooperation by Wayfarers:

The pact would start with members of the three varieties of Christianity affirming their right to disagree with one another, both in the sense of holding different convictions and of speaking publicly against the errors of the other parties. In this way, an individual Christian would retain his sense of integrity and his spirit, both of which would be weakened by too much ecumenism.

But signatories of the pact would also pledge not to attack the other parties in the sense of trying to cause harm. They would identify Modernism as the greater enemy.

–from the Orthosphere.  Although I think “Modernism” is a mistaken identification of the enemy.  Not completely wrong, but not on top of the target either.

*Arbitary in light of the ends in view.  Not arbitrary for every purpose, naturally not.