Ogata,
he's the only one from the original who NEVER got to reprise his role. I'm excluding dr Yamane's (refeatured in GRA's) adopted son Shichiki? because his kids at least tell us what happened to him (GvD). Emiko (GvD) and even Steve Martin (G85) if you want to consider KoTM also got to reprise their roles. The actor's been in many Toho films since. Maybe he'll finally get to tell us what happened to him in the timeline of the new film.
Yuki,
as much as people hate GvSG, this guy and his antics were the highlight of the human action in the film. A real tough guy.
Commander Kuroki,
the young JSDF leader from GvBiollante. Had a ton of potential, though he was rather Sam Eagle like, kind of reminded me of my older brother. I'd have liked to see how he turned out. Then again he was featured in many of the comics for the hesei films. Who is the Super X 3 Pilot in GvDestroyah? ahh just looked it up, it was him
http://en.wikipedia.org/wiki/Super_X . still I feel like by this point he should have been running the show, good thing for him he didnt die.
Mamagon,
well the character she was based on...and the corn eating hippie...yeah as said above Gigan had an entertaining cast.
Glen,
Nick Adams RIP was great in this film. "You stinkin rats!!!!" I should start yelling that when I lose at craps at the casino. Also kind of curious what became of Tetsui and his inventions, did he impress Oni san enough to marry his sister?
The Girl from Mothra's Island in GvSea Monster. Hot. The doctor's daughter from SoG as well. Curious what happened to her and the Solgell Spinach reporter. They had chemistry. Would've been interesting to see them study Minya on Monster Island.
The cast of KKvG from the coin flipping to the corn aching to tarzan that movie's cast is entertaining in either format.
Aliens well we saw the Xillians again and their old commander in GFW (well technically a new role) but if Showa could continue, that open ended escape into the future line piqued my curiosity.
Either of the commanders from the Black Hole.
Emi Kano. Badass Futurian. Would have liked to have seen what the future looks like besides just the bottom of the ocean, but I suppose the budget couldn't justify it. Thats it for now.