Thank you all so much for the kind words! Usually when I make post like that they don't get much attention, so this came as a bit of a surprise.
In regards to the merger, I completely understand the desire to see Vanitas become his own person, and it would actually fit with the series' themes of individuality and ability to change. For a long time, I was actually advocating for the same thing, hence why I made this thread a while back. But after going through BBS and analyzing Ventus' arc, it really seemed like the only way for things to truly come full circle was via their reunification. More specifically, remember how I kept bringing up how Ven's arc in BBS was essentially to regain the part of himself that he lost? I was actually reffering to this scene:
In it, after Sora touches Ventus' heart and prevents any further damage, he says that one day Ventus may be able to "win back the part he already lost", indicating that his journey would ultimately be about mending the damage Xehanort did to him, and becoming whole once again. Meanwhile, Vanitas has this same goal, but taken to an extreme. He's based his entire existence around filling the void in his heart and becoming whole once again, and along the way, due to Xehanort's influence, he lost his humanity and sense of individual worth. It just seemed perfectly fitting that the two would eventually rejoin on their own terms, and gain all the things they were looking for in the process.
Now, with that being said, there is definitely a way you can cleanly wrap up these lines and plot beats in a satisfying way while keeping the two separate, and that would be by shifting the framing so that it's more metaphorical than literal.
First, rather than have Vanitas on the verge of death, simply put him on his last legs, where he sees his defeat as inevitable and thus the scene I proposed earlier can still play out in a similar way.
In terms of being consistent with the set up back in BBS, "Winning back the part you already lost" could also translate to winning it back from Xehanort's grasp, which would be represented by Vani joining Ventus' side and turning against Xehanort in the finale. So in a sense, they now both have eachother in theie lives again to give them closure, rather than simply rejoining entirely. Their reconciliation and choice of a fresh start would then be what triggers their hearts to grow complete and independent, so neither are pure light or pure darkness anymore, but their own people with their own light and darkness (perhaps this could show that hearts have the power to choose what they are on just as much a physical level as an emotional). This way, Vanitas could still regain his humanity and Ven could still become whole, which allows them to both complete their arcs without changing the overall trajectory of them.
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In general, though I would personally prefer them merging and think it opens the door for a more unique character, having a scenario like this would still be almost just as conclusive and satisfying (the only real lingering thing would be Vanitas' road to redemption, and the soul searching he'd have to go on to better define his new purpose in the world).
I think a merger would really only work
better if KH3 was always intended to be the last that we saw of Vanitas, since at least this way he'd live on through a unified being whom shared his experiences, suffering, ect. If it's not, and there's room in the narrative for Vani to persist and go on his own journey, then I think the second idea, with the two staying separate and becoming whole via their new bond, would be more appropriate. Either way, Vanitas definitely deserved a second chance, or at least better closure in some form.