Here's some I'm vouching for.
Princess Knight - I have the whole series in the bilingual Kodansha version, but it's impossible to come by outside of Japan. This was the first shoujo manga (it's even mentioned in the liner notes of the English "Revolutionary Girl Utena") and is very important as such. It's also an example of the clear tie between Disney and "manga/anime-style," and would appeal to both manga/anime and Disney fans. This series should definitely be published in the USA...
Jungle Emperor Leo -
1. This had a following back in the day (like Astro Boy, which Dark Horse already picked up)
2. Disney ripped this off (so it connects to the Tezuka copying Disney style - Disney copying Tezuka thing I mentioned earlier)
3. It's cute and kids would buy it...and lots of merchandise
4. It's environmentally conscious!
Shintakarajima/New Treasure Island - Dark Horse should pick this up, seeing as they tend to go for the more "vintage-1940s" type Tezuka stories. This is Tezuka's first big manga and is a very important precedent.
Alabaster - Vertical already went for MW and more of Tezuka's serious works, they should go for this one, an important bridge between Rock (Vampires), a kids-story villain, and Yuki (MW), a totally evil "adult" villain.
Ayako, Swallowing the Earth, and Human Metamorphosis for the same reasons as Alabaster. They are more "adult" Tezuka works.
The Vampires - of course. This is probably my favorite Tezuka series to date (I have all of it in Japanese). It fits in with MW and Alabaster as I said earlier, so Vertical may pick it up. It also seems "retro" enough that Dark Horse might do it also....
Of course there is the problem of it being unfinished...but in that case maybe they can just publish the first story arc. [/b]