In other words I don't think that Cloud's choice of sword is that big a deal: It is what the player chooses. Outside of that the Buster Sword is iconic which is why it's Cloud's sword of choice for both the flashback above and for Cloud's crossover appearances in Kingdom Hearts and in mobile games
Also, that one-in-one is a spiritual battle. The physical weapon Cloud may have had would be irrelevant. It's only a visualization and projection for his spirit to use.
And honestly, Cloud's Buster Sword isn't the only iconic weapon of his. The Ultima Weapon is showcased in the Dissidia series along with the Forcestealer. Those swords certainly have permanence and notoriety among FF fans as well. The Buster Sword is the most iconic but not the only one.
It wasn't the strongest weapon in the original. But that doesn't necessarily mean that has to also be true in the remake. Things are not going to be 1:1 identical between the two. And we don't know what parts of CC's story aside from the Nibelheim incident will be included in the remake's canon, if any, so we don't know if we'll know if Zack saw the sword in any particular way or if Angeal ever even existed in the remake's canon. There's no reason Buster can't at least be upgradable even if it's not the strongest. The mechanics of the original need not exclusively define how things like this work in the remake.
If the Buster Sword can be upgraded ala the Kingdom Key in KH3 and allowed to be relevant
that way, not as the strongest weapon but a solid well-rounded weapon with a unique and helpful passive ability, that would be cool and acceptable.
But it would be a glaring and unnecessary change which needlessly flies in the face of the OG, to shoehorn the Buster Sword as the strongest weapon of all. It simply isn't,
It never was. It was Zack's final weapon and
Cloud isn't Zack.
Crisis Core shouldn't be changing the reality of the game because this isn't Zack's story nor a sole testament to him. The weapon Cloud picks up after killing Ultima Weapon is his
Ultimate Weapon. A blade of energy obtained from slaying one of the strongest monsters is Cloud's strongest sword. Each character finding their strongest weapon with unique and powerful properties and appearances is one of the most iconic elements of FFVII. That doesn't get to be replaced for Cloud because of Zack.