The multilateral approach to Buddhism will never work. Nichiren Daishonin taught this. We know this because we believe Nichiren Daishonin and we see with our own eyes the disunity of mulitlateralism. The last and most important of the Five Cardinal Sins is breaking the unity of the Sangha.
Multilateralism in Buddhism is antithetical to Buddhism. The question remains, to what can all Buddhists unite? I have heard it said that Buddhists can unite, "around the Three Treasures" but almost to a man, everyone upholds a different view of the Three Treasures. Among the Theravadins, ask one what is the Treasure of the Dharma and he will tell you: "It is the Pali Canon.", ask another, "It is the Four Noble Truths, and another, "The Eightfold Path.", and yet another, "It is the Twelve Linked Chain of Causation." Ask a provisional Mahayanist and you get an even greater variety of answers. This is equally true about the other Two Treasures. If there is nothing to which we can unify than the Buddha is a great liar and there is no Cardinal Sin of Disrupting the Unity of Believers.
The Three Treasures to which all Buddhists should unite are the most profound Three Treasures: Namu Myoho renge kyo, The Master of Teachings Lord Shakya of the Original Doctrine of the Lotus Sutra (Gohonzon), and the Sangha led by Bodhisattva Emminent Conduct (Superior Practices).