If Jesus’ prayer (John 17) that believers would be one as he is one with the Father is to be answered in this present life beyond the oneness we have by the Spirit, there will need to be an emphasis put on finding common ground in adherence to the gospel. There are all sorts of differences which will likely remain to some extent, but if the common ground we find in the gospel can help break down those differences so that they are so secondary as to be essentially beside the point, we will have come a long way towards complete unity.
There are admittedly daunting complications, and such a thought as this post is musing on would seem noble to most, but at best naive to many. I do think churches need to be willing to submit what differences they have to the test of scripture, tradition and reason, probably adding experience as a confirming factor by the Spirit in that.
As Paul suggests in his first letter to the Corinthian church (1 Corinthians 11), differences in this present life among professing believers and churches seem inevitable. In that context it was the Lord’s Table that was being referred to, even a meal. Prior to that and in the context of that letter, it seems that there were serious divisions which threatened to undo their unity in Christ. Paul of course addresses that in the letter. So this is likely to be an ongoing concern forever in this present life.
Again we need to major on majors and especially the major point, the gospel/good news in Jesus and look at everything in relation to that. In doing so, hopefully we can learn to set what differences we have more and more aside, and even agree to live with many of them. Because of the common unity we have in the faith in and through our Lord. Something to which we ought to aspire even if its full achievement seems impossible.