The Martian Manhunter is on his second Showcase volume. I don't recall it being much better in color, so I'd give it a shot if you're at all interested.
I've never understood the draw for Green Arrow, personally, but would love to see more of the early Aquaman. But I can see why DC would stay away, since they've spent decades showing us how he's not really a worthwhile property.
This may be the fact that my earliest exposure to the Golden Age was All-Star Squadron, but I definitely second Robotman (especially if they could find an editor who'd be willing and able to talk about the Binders' Adam Link stories in context--actually, context is what the DC Archives tend to ignore, which is a shame), and would also appreciate Tarantula and Johnny Quick, who've both impressed me quite a bit in the few original stories I've seen.
Generally, I tend to think that the rule for collections should be that the material isn't widely available otherwise, though. So I wouldn't mind seeing anything from Slam Bradley to the Inferior 5, whereas I have no idea who greenlit the Outsiders Showcase.