There are some good aspects of this idea, such as how Stitch was tied into the storyline, but I wouldn't want Leon to show up as a mere cameo. That's exactly the kind of integration I wouldn't want to see in KH3, and Radiant Garden on the whole is important enough to warrant at least as much attention and resources as Twilight Town; I would argue that it deserved far more given the sheer centrality of it to the story arcs of so many characters. At this point, KH3 is going to have to go out of its way to explain and work around problems like why the Restoration Committee isn't working with Ienzo (and possibly AtW) to help Sora. Or why Lea and Isa are meeting at the Twilight Town clock tower, a place that holds no meaning whatsoever for Isa as a character. Or how, indeed, Aqua will regain her armor if she doesn't get it from the Chamber of Repose under Ansem the Wise's lab. It's not just that the absence of Radiant Garden is a bummer, it actually degrades the continuity and emotional logic of this entry within the context of the whole series.
Speaking of Stitch, how nice is it that KH3 gets neither Destiny Islands or Hawai'i. Used to be the general, accepted consensus that we would only get one, and we all made peace with it, but hey. I guess we really can't have everything!
KH3 isn't even out yet, trying to justify its errors in composition on the imagined merits of the next Kingdom Hearts game is not persuasive.
I haven't seen any suggestions for an FF-specific subplot in Radiant Garden to distract from the main overarching story, that's your inference and yours alone. On the contrary, the world of Radiant Garden has a higher degree of relevance going beyond the FF characters because of the decisions Nomura made in terms of the narrative between KH2 and KH3. Like I said: if it's about "original characters" and prioritizing their arcs, cutting out Radiant Garden is totally counterproductive.
I'm curious to know, then, how you justify Nomura's decision to bring it back in Birth by Sleep for the distinctive purpose of establishing it as the home world of 50% of the Organization's members and ensuring that Terra, Aqua and Ven all have links to the world.
One could argue that every arc was closed in KH2. Nomura decided to make more games reopening them. That's where the criticism around this choice stems from, for me.