Such a difficult question....there are a bunch of things I could get nitpicky about and say I'd change like kh2 sora's seeming lack of intelligence or Terra's gullibility but honestly kinda feel that they are best left as they are. Changing them all to much the way I'd prefer seems like it'd ruin the story I've come to enjoy enough to stick with despite all my criticism.
Regarding Terra (Aqua and Ven to an extent as well) I often get the vibe that they get much more flak from some fans than is due. Like the so often cited "dumbassness" of Terra is not really that problematic because, well, Terra does NOT have the profound knowledge about certain individuals that the audience has.
We, the audience, know that Maleficent and Hook are villains, but how is Terra supposed to know at first sight? Not to mention he and the other two also had not ever (in Ven's case) or very rarely (probably in Terra's and Aqua's case) left the Land of Departure, living a very sheltered life without much outside interactions.
If anything, Terra should be applauded because he does something that even people in real life rarely do: He doesn't judge a book by its cover and just because Maleficent and Master Xehanort wear black he doesn't immediately put them into "evil" category.
And with Queen Grimhilde he actually noticed pretty fast that something was "off" and he actually told her off at the end, that her so-called "radiance" is nothing but poisoned jealousy and vanity.
I wonder sometimes though why this only looking on with audience's eyes often only applies to the wayfinder-trio and not the other two trios as well. Riku in KH 1, Sora in KH2 as well as Axel, Roxas
and Xion in Days all to some pretty stupid things and mess up, yet they do not get nearly as much flak on theirs as Terra gets.
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On the topic in general, I won't say that they should ditch/kill off any character I just don't like because I personally do not count this as a legit reason.
On the three main trios, I'd say they all have more potential that could be explored and developed over time, so I think they shouldn't ditch either RLX or TAV, since they each only had one game for them so far. Out of the SRK-group, Kairi definitely needs the most attention since as of now, when I compare all three main trios, I get the impression that the SRK-one is the most imbalanced in terms of how the characters are developed (not necessarily in screentime, just overall development).
When looking at all nine main OCs (ten if you count Naminé), I do have to say though that despite how much I like Lea/Axel and Riku, they should take a backseat for a while or at least get some less focus, as out of all main OCs they have had the most exposure and development so far, they do not need much more, at least not until some other characters can catch up.
Then there is of course the issue with having the main OCs actually
break out of their trio structure and intermingle with the other OCs and Disney characters. There are the obvious cases like Riku and Terra, Kairi and Aqua and of course Sora and Ventus, yet as with those there has been much emphasis put on their
connections and how they are tied together by fate et cetera, but actual interaction is so far (except on Aqua's side), close to nil.
Second with these "pairings" (and no, this has nothing to do with shipping) is, that when we look at those we can easily spot the many similarities, so one smart move might actually be to put some more focus on their
differences, even if they are only small quirks and traits, they can help flesh out the characters more.
The not so obvious cases are of course the other mixups which promise some interesting insights, like i.e. we already had a whole thread about Roxas and Ventus meeting, or Naminé meeting Xion again and many more. This (again) holds of course especially true for Kairi, as she desperately needs to interact with characters other than Sora and establish a true place within the main cast that the audience can
see and that goes beyond just being a PoH and the inserted obligatory main female love interest for shipping purposes.
The tidbits we saw in KH 2 with Kairi, Hayner, Pence and Olette were actually a good start, but Kairi definitely needs more like that, also with the other main OCs and MDG as well as maybe the other Princesses.
On the topic of the supporting OCs, I'd say some major ones like Ansem the Wise, Ienzo, Even or Dilan can use some more interaction with people outside the main cast, possibly FF cameos and Disney characters at Radiant Garden like Scrooge McDuck.
The others though, who aren't really important to the story, should have only cameos at best or otherwise be included in the credits.
Like i.e. having Marluxias human form run a flower shop in some world or something.