Since writing this answer I have realized my initial response of Abraham as the one being referred to in verse 11 was incorrect. He is surely included in the fatherhood of the ecclesia a few verses later, but here the "one" must be Father God. The determining factor that changed my mind was the OT Scriptures that follow.
In the first one, quoting Psalm 22:22, Jesus is prophesied as saying, "'I will proclaim Your name to my brethren, in the midst of the congregation (ecclesia) I will sing your praise. And again, 'I will put my trust in him.' And again, 'Behold, I and the children God has given me [are for signs and wonders in Israel]'.(Isa 8:17,18)"
We know Jesus did not proclaim Adam's name (the other possibility) who got us all into this mess we are in. And although he honored Abraham as the one to whom the promises were made, he did not "proclaim" his name or "sing his praise" in a worshipful sense. Nor did he put "his trust in him." All this is speaking of the Father which the last quote verbalizes when it says the children (brethren) are God's. So we are all children of God first and foremost.
However, I believe the text goes on to reveal that Abraham is also our father from a human standpoint. That is, he is the father of all those who are called "brethren," which would be the whole ecclesia. In verses 14-17 we learn that Jesus had to be made "like his brethren in all things." Including lineage from the fatherhood of Abraham. And that his descendants alone are the benefit of his ministry. "For assuredly he (Jesus) does not give help to angels, but he gives help to the descendants of Abraham.
Now the reason Abraham is routinely rejected by most believers as being the human father of the ecclesia is because we know that not ALL who make up the body of believers are Jewish. Some are non-Jews. The so-called "gentiles." How can we be physical descendants of Abraham? Because we don't know our history and forget 5/6 of the nation was scattered among the nations in 722BC as recorded in 2 Kings 17,18. That's why many teachers who study the Book of Hebrews come to the erroneous conclusion that it was written ONLY to Jewish believers, and not to "gentile" believers as well. But we non-Jews are declared Abraham's seed also. "For if you belong to Messiah, you ARE Abraham's seed, heirs according to promise" (Gal 5:1)
The inescapable conclusion I have reached is that the whole letter to the Hebrews is addressed to ALL believers, Jew and non-Jew. And therefore ALL believers must be Hebrews, true physical descendants of Abraham. Not just spiritual "seed," but the actual chosen seed as we read in Galatians 4:28: "But you brethren (addressed to Jewish and non-Jewish believers in Galatia), like Isaac, are children of promise." Being the physical seed of Abraham doesn't actually gain us anything, as Ishmael and Esau discovered, unless you are a child of promise. We must be chosen "in him [Jesus} before the foundation of the world" (Eph.1:4) to be the "brethren" of the Lord. And children of God.