In Colossians 4:16 Paul mentions a letter he wrote to the Laodicean church. If this letter were found today, should it be included in the canon of scriptures?
Yes. However, it would have to be so sure that we would have no less doubt about it than we do the letters that are there now. I'm not sure how that would happen. I don't think it's going to happen.
The reason I say "Yes" is not because everything the Apostle Paul said was inspired. When he was in his teaching and writing his letters for the churches, he was in an office at that moment, experiencing the Holy Spirit.
Not everything an apostle said was inspired. We know that from Galatians 2 where Peter blew it and had to be reprimanded by the Apostle Paul because of the way he was behaving. And probably some of the things he said as he walked out of that room were not very helpful.
So not everything an apostle said was inspired. It's their writings that are inspired. He said that that letter was his. He said that it should be read. And therefore he was treating it like one of his letters. That's why I say, Yes.
But I don't expect it. I hope nobody is motivated to go out and try to forge it. And I don't think it would contribute anything new to what we have. I don't think we're missing out on the sufficiency of God's revelation to his church for what we need to live lives pleasing to him.