I accept from the get go that this might appear initially a radical theory, but I ask people to hear me out and I'll explain. We know that the combined 20 people are needed to forge the X Blade. However, will they be our only participants? Actually, the evidence suggests not.
I'll first refer us to the Xehanort Report V:
'few Keyblade wielders remain now; I have heard of but a handful of others outside my circle. But the world is vast and more of our kind may be out there.'
Let's analyse this, I believe that Xehnort's circle is himself, Eraqus and Yen sid and consequently their apprentices. Who could the others be? I believe they are relics of the dandelions, either apprentices or the Dandelions themselves as we know that both Ven and Marluxia are Fairy Tale characters. How they 'awoke' from the Unchained Realm I don't know, I guess we need more UX Plot.
Now why would they appear? Let us refer to something Vanitas says:
'Then Keyblade- bearing warriors will flock here from each and every one of them (worlds), to battle for for the light within Kingdom Hearts! And just like the legend says, the Keyblade War will begin!'
If we believe this to be true, then why would they appear for the BBS war and not the KH3 war? The criteria would be fulfilled, the X Blade forged, Kingdom hearts unlocked and thus the keyblade war started. So it's logical that these warriors will appear for the Keyblade War.
These unknowns would also explain a few odd elements of the plot, such as Triton knowing about the Keyblade and Genie hearing about the keyhole.
Lastly and this is more speculation, but Square ran a competition a little while ago about being included in KH3, now for 'plot purposes' this wasn't revealed. Now many, including myself thought ' our names will be on the keyblades in the graveyard'. However, what if these people are the unknowns ( or at least a proportion) who will be participants in the Keyblade war?
To conclude, there are more keyblade wielders in the world than we are aware of, if unlocking Kingdom Hearts attracts these warriors, then I believe that the final war will not simply be our 20 warriors, but a large number of warriors.
Perhaps, this large scale battle is what Nomura meant when he referred to the ending? It would be unexpected, unlike what we've seen in the series before and would be a genuine war.