Can you explain what you mean by this? I'm a bit confused.
By the time TAV were introduced there were already many important characters and plot points in place. BbS being a prequel, Nomura and co. were met with the odd task of implementing these new important characters and plot points as if they were always part of the mythos. Since there's already a preestablished mythos and TAV are supposed to effect what is currently happening in the story, they're given relevance by attaching them to what has been preestablished. This is a tricky task to undertake with any prequel, unless it's planned from the beginning, otherwise it does come off as tacked on. Which is the issue with TAV. Half the time they're involvement isn't because they're legitimacy important, it's because they need something important to do so they're involved.
That means they either have to:
A) Be involved in a resolution that
already had a resolution and didn't need further elaboration.
B) Fill in plot holes that no one asked to be filled or could have been answered in a much simpler way and still make as much sense.
C) Retcon
To name a few instances off, there's the issue of Kairi and Riku receiving The Keyblade Inheritance Ceremony. In fact, the whole concept of the ceremony is unnecessary, it's a plot device implemented to try and make deeper connections between characters. It was
already established that a Keyblade chooses its wielder when it sees a person as worthy through having certain traits like a really strong heart. There really was no need to elaborate on it further. The KIC was introduced to make Terra and Aqua seem more important via connecting them to Riku and Kairi.
Aqua creating CO. LoD was implanted with a defense mechanism in case the forces of darkness invade to prevent it from taking over. A defense mechanism that's
always off. Having a defense mechanism that functions this way makes no sense and like the KIC and Kairi, plot wise, the reason it's set up this way is to make Aqua more important and more involved with something that didn't need it. I will give it this, connecting Aqua to CO isn't a bad idea, but it needed a few tune-ups. It would have been an easy fix by saying that LoD transformed automatically when it sensed impending darkness, but Master Keeper allows its wielder to manipulate it. As it is, it's a bumpy way to add relevance.
Ven's connection to Sora and Roxas. Ven really does get the brunt end as his connection to Sora and Roxas is almost completely arbitrary. Ven's involvement is mainly to answer two things: why Sora and Roxas can dual wield and why Roxas looks different from Sora. It was already hinted at in KH2 that Roxas had a heart and 3D pretty much confirms it. Why they can dual wield would have been as simple as saying that Roxas grew his own heart, and with it, gained his own Keyblade. It was also already established that Nobodies tend to look slightly different from their Somebodies. Nomura even went out of his way to say, yes, he drew Roxas with the same face as Sora, the only thing he changed was the eyes. Now there's this big Sora/Roxas/Ven mess that didn't need to happen, but Ven needed to be important somehow.
Kairi being sent to DI. Yes, I used that forbidden word, "retcon", but I'm not sure what else to call it. It was already established in KH1FM that Ansem sent Kairi out to look for the Keyblade and that she was led to it because she's a PoH, who have a special connection to the them. As the series progresses we see that this is true. Their hearts together form a special Keyblade, they can be used as ingredients for the X-Blade, and Keyblade wielders just so happen to be attracted to them and bump into them at the right places at the right times (like both Aqua and Mickey both happening by while Kairi is being attacked and Terra finding Cinderella and Snow White just as they're in turmoil/danger). There's no reason why Kairi getting to DI should've had something to do with Aqua casting a spell on her. Or that the spell should've been used in this way or cast on her to begin with. Like other things, how Kairi got to DI was something that was
already answered and no one questioned otherwise nor did it need to be changed.
There's also a few other oddities here and there that stem from BbS being a prequel, like I think Peter Pan claimed to have never seen a Keyblade before in KH1 even though he met TAV and saw them wielding them and even had Ven's toy Keyblade in his possession for a while. Neither Mickey nor Yen Sid mention that some comrades went missing a few years ago and don't think to ask Sora and Riku for help or if they know anything, knowing full well that the Keyblade wielding ability is normally inherited and Keyblades are extremely rare. Hell, Mickey even asked
Vanitas of all people where he got his because he knows that only masters and those on master level can give the ceremony, and that's a really small pool. In fact, there are a bunch of characters that met TAV and saw their Keyblades yet never bring them up.
It comes with the prequel territory, but yeah, TAV's implementation tends to be clumsy.