Put in a spoiler box because it's a wall of text that might scare people away XD
They could go about the roundabout way to it, that Axel's Somebody might trigger something with Roxas and with the Ven-factor the whole thing'll be worthwhile.
The problem is and I'm hardly saying I'm exempt from this, is that fans want their characters to make an appearance, and 90% of the people don't really care about the plot to the point they'll actually stop and think about whether or not character A making a comeback would mean anything, if it'll even make sense.
At least I have Nomura implications, the Ven-factor in and of itself, Castle Oblivion and the KH2 ending to back up Namine being probably up for a comeback. Axel's Somebody, as well as just about any other Org Somebody (other than the first 6 members') might make for a nice cameo, but if they'll show up, they won't matter.
What frustrates me the most about the Organization is that people don't really know them, so they end up falling in-love with the personalizations and characterizations they set up themselves for it :\ part of why I adore to death the CoM members (or am amused by them greatly, as is the case with the members shown in Reverse-Rebirth) and why I couldn't care less for the most part for the KH2 members - CoM members had their characters portrayed. The KH2 members didn't. While I do tend to hype over characters I don't really know, it's short lived. Had Sora not been a character I could relate to and analyse to death, I wouldn't have still to this very day considered him to be one of the best characters in the game and one of my favorites, if not the favorite in the long run. Hence, while I did start with somewhat liking Kairi - it was short lived.
Most people don't see things this way though and would sooner create a character for them to like instead of seeing that there's nothing there (like with Kairi XD). Yes, I "create" characters for the ones I like, but there's a difference between conjuring an entire back-story, to simply constructing a view on the character based on what was displayed in the games.
I make Sora into a ticking time-bomb of angst, not have Demyx be a merman :\