What if...

I would continue on in the faith just as I do now and consider the manuscripts to be in the same category as the "gospel of Thomas" or "the gospel of Mary". Total fakes.

 

If they did not contain much of the scriptures we have today, and didn't mention many essential doctrines, I would consider them to be incomplete manuscripts.

"Absence of evidence is not evidence of absence."

I'm solidly convinced what we have now is inspired and complete so anything else to come along that didn't confirm current scriptures I would reject.

I would rely on the Holy Spirit to guide me and from that guidance I know that what we have now is valid.

Romans 8:16 The Spirit Himself bears witness with our spirit that we are children of God,

 

The fly in the ointment of the OP question is that we can reproduce the entirety of the NT from the writings of the (very) early Church Fathers and there is no additional writings, nor lack thereof, to consider. Our Bible is both reliable, accurate, and because of its Author -- "authoritative" to shape our lives, belief, and faith.

 

The difficulty with this question is that it would mean the earliest texts (almost extant) documents of the NT derivate so significantly from the texts of one or two generations later that it wouldn't make good sense.

Right now textual critical scholars have done remarkably well to isolate the text of the NT to about 99.9995% accuracy given recent (and ongoing) textual discoveries. For earlier documents to appear which would dramatically change the readings of the CT would be so surprising it would throw the entire text of the Scriptures into question. The historical work that has been done by the most reputable scholars has really brought the autographs into better focus.

Given that the majority, the vast majority, of the textual critical issues in the NT are related to minor issues in spelling, substitution, or transposition (none of which affect doctrine or teaching) there aren't conceivably major changes forthcoming in any textual find. If there was it would throw that find into question rather than the critical text of the NT.

The amount of work that has been done in the past several generations, specifically through the reconstruction of the CT, has done more to qualify the text than ever before. I just don't see something like you're describing being possible. Given the insurmountable evidence to the doctrines you're talking about (resurrection, second coming) in early church historical attestation as well as textual attestation I don't see it as likely.

If you go the earliest secondary sources to the NT, stuff like Papias or Clement I or Ignatius which are all pre-AD 100 you just don't see a doctrinal derivation like you are suggesting could happen. I could go on but I think the point has been made. (My PhD was in patristic theology so I've got a pretty good handle on this era and evidences)

Even if it did happen it would likely throw the find into more question than the existing text.

 

The hypothetical situation being presented by the OP was in regards to omissions in doctrine, not contradictions with the readings of current manuscripts. I don't see how something not appearing in these hypothetical older manuscripts would throw the entire texts of existing scriptures into question.

 

The doctrines spoken of the OP, if different or missing from the autographs, would be surprising to say the least. My point is that in such a (hypothetical) case the questions would be more on the "discovered text" than the existing texts.

The vexing dilemma of the OP is that the existing text is so historically robust that any change or substantial omission would be remarkable.