If we understand that angels and men are created differently and for different purposes in relation to God (and I think we have to understand this, given the biblical testimony), then we have to be willing to rethink what it is that we believe about eternity.
Specifically, men do not become angels.
This is a difficult one for many Christians because the church has seemed to have this dominant narrative that once we die, we "get our wings." We straighten our halos and become angels. We have all of these images of winged, haloed men sitting on clouds and becoming the messengers of God back and forth between heaven and earth. We even have certain sayings, particularly when someone young or exceptionally good-hearted dies. "God must've needed another angel."
But to believe this is to severely misunderstand...well, a lot of things. It is to misunderstand the nature of man, which is to be the glory of God. It is to misunderstand redemption, which is to glorify us in the presence of God. It is to misunderstand God's design for His creation, which is to fulfill humanity, not to transform it. (If God had wanted men to be angels, He would have simply made angels and not men. Adam could just as easily have been created in the service of God, but he was not; he was created for the glory of Him.) And it is to misunderstand the nature of angels, who are fundamentally different beings than men, even particularly good-hearted men.
Couldn't it be true, though, that there are two classes of angels - the heavenly beings created by God and the redeemed souls of good-hearted men? With God, all things are possible, but we have no evidence for this one. And how many of us would be satisfied to labor our lives away in love, trying to glorify God the Father, only to get to Heaven and be put in the service of Him rather than recreated in the glory of Him? How many of us would rejoice to discover that the very thing God had spent our lives telling us was not so true after all? It would be the ultimate bait-and-switch, and if that's the case, what are we supposed to believe about this God of ours? So it's possible, yes, but it creates more problems than it solves.
Not to mention that nowhere else do we see God having created two classes of anything. There is one heaven, one earth. One light to guide the day, one light to guide the night. One mankind. There is simply no evidence to suggest that God deals in dualities, in multiple classes of the same object.
And then, of course, there's this: when Jesus was being tempted in the wilderness, He said to the Tempter, "I could call down legions of angels to help me if I wanted." Again, what is the nature of angels? Are they simply the redeemed, good-hearted men? That's a bit weird here, don't you think? Jesus, the Son of God, the very divine being Himself, would call on the best of men to help Him in His struggle against the powers of darkness? That creates so many theological problems that it's absurd. It places men equal to or above God. It removes the power of light and darkness from the heavenly realm and puts it in the human ones. It reflects an image of God that requires the help of men. How many more do I need to list here?
There is a fundamental difference between men and angels, and it is one that does not magically or mysteriously disappear once men have died. We were created for the glory of Him, and in Him, we will be glorified. We will not become angels.
So we have to stop thinking like this. We have to stop letting ourselves get wrapped up in images of halos and wings. That's not what God intended for us; that is not our eternity. Will there be angels in Heaven? Absolutely. The Scriptures tell us of the heavenly host.
But they're still angels, not saints.