I think this would be nearly impossible since, as has been stated, Hasbro pretty much got it right the first time around.
Yes, Horror Show could have been bigger, but you'd need an entirely new set of molds to correct that matter, and that would be horrifically expensive.
About all I can think of that could be changed would be to recolor Daina in her gold-and-green camouflage flight suit, and maybe make a new head where she's wearing the pilot's helmet instead of the fur hat, and maybe do Schrage "as is", but with a light green uniform instead of the tan.
Short of creating new characters (and no, that's not a plug for my customs)

, or dressing up the characters in significantly new uniforms, as they did with Dragonsky in 2005, I just don't think an Oktober Guard Convention set is all that workable.
What might be workable is a Convention Set of either previously unseen versions of popular characters, such as the Daina and Schrage I just mentioned, along with other characters from the comics and animated series that never made it into figure form. Unfortunately, a collection like that would be so diverse (and likely difficult to pull together based on existing parts), that it would be pretty hard to come up with a "theme" for it.
Although, I'll admit I wouldn't mind seeing a couple of Oktober Guard vehicles.
