The conjecture that the "signs" in John 20 are the entirety of the teaching that the Church received from the Holy Ghost as promised in John 14 is yours. It is not in the scripture, and in fact is contradicted by the wording and context of John 14, where Christ speaks of teaching rather than of signs, that is, miracles and actions.
Further, the purpose of the gospel as stated in John 20:31 is to furnish proof of divinity of Christ; it does not say that everything necessary for the Christian faith formation, "for the salvation of man and fellowship with Him" is contained in it. At any rate, you asked if revelation exists outside of scripture, and John 20 and 21 directly state that yes, there was such revelation given the Apostles.
" At any rate, you asked if revelation exists outside of scripture, and John 20 and 21 directly state that yes, there was such revelation given the Apostles."
No, what I said was, "Nowhere does the scripture say that there is a revelation outside of scripture that reveals God's plan of salvation for man or how sinful man can have restored fellowship with Him." There is none. That is what John is saying in John 20:31, "But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name."
There are many things, as John says that are not written, but what is revealed is God's plan of salvation and as Paul states in Gal. 1:6-9, "I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed."