One Piece airing on Toonami would be pretty cool. While I'm not a huge fan of the series, what I saw from the first few uncut DVDs and the episodes that did air on Cartoon Network after Funimation took over was pretty good and I'd be interested in seeing it on TV. Airing from the start would probably be more helpful in getting people familiar with the characters, setting and storyline, especially when many people are probably only familiar with how 4Kids handled those first few arcs. Though, I think that One Piece would need to have a weekday or weeknight slot instead of airing one or two episodes on a weekly block like Toonami. Like you mentioned, having two new episodes would take a slot away from something else and having one series take up an hour out of a three hour block would be problematic. A weekday or weeknight airing would allow for more episodes to air, more progression for the story than a weekly block could bring and as a result, more people could possibly become interested in the series. I'd like to see One Piece on Toonami, but it would be kind of difficult.
Personally, I'm not sure if One Piece would have been anywhere near as successful in the U.S. like it is in Japan if 4Kids didn't pick it up. Their dub certainly hurt its chances, but even without that dub, I don't think it would have that much more of a chance. It probably would have been more popular, but I don't think that it would have made a significant different, especially when it did take One Piece a good few years to get so popular in Japan as well.
A weekday slot does seem like the best possibility. I'm just not sure it would fit very well with the other programming there. Which is another problem.
I in fact do believe that One Piece would be a
much bigger succes in US if it has gotten a good dub from the start. I'm not even doubting. In Japan, the manga is selling almost 6 times as much as the manga closest to it, and the anime is one of biggest there too, though I don't have any specific facts to that one. If you look at it, Naruto is pretty much just bigger the places that One Piece was introduced with 4Kids in. I'm not saying that a better dub from the beginning would change that very much at least, but it's interesting to note anyways.
Also, 4Kids changes One Piece so much in so many ridiculous ways like lolipops taking cigarettes place instead of just removing them, seriously using water guns, and even removing Luffy's big happy smiles which I don't even see the problem with. These barely begins to descripe the long list of changes like that. They also skipped entire arcs leaving serious plot holes and it all really makes the big changes they did in stuff like Yu-Gi-Oh go unnoticeable in compariation. One Piece would obviously have been much bigger.
What would people call the big animes in the US? Just to name some; Pokemon, Dragon Ball, Gundam, Sailor Moon, Naruto, Yu-Gi-Oh!, Astro Boy. In Japan the difference would pretty much just be adding One Piece to that bunch, and it has done better than most of them. So even if it wouldn't have been just as big a succes, you know that their tastes really doesn't seem to be that different.
Well, I'm going a bit off-topic maybe. I actually think there's serveral Shounen Jump adaptions that would be a great addition to Toonami, some of them in need of a dub before it could happen. Death Note is a big hit, and not
that long. The new Hunter X Hunter seires is another one. It might also please some of the fans that want Yu Yu Hakusho back. As I've previously mentioned Toriko looks like it has a great future ahead in Japan, so we should see how the rest of the world likes it. The latter two of these are series that could go on for quite a while, and we can't really have a lot of those since combined they take too much space imo. So maybe somthing shorter like Medaka Box that just made its anime debut this spring.