@Clifford:
2. Dressrosa costumes
I know the canon visuals in the opening (obviously) are in the Dressrosa costumes, but the rest of the opening uses old costumes from like 5 years ago. I don't know why they continue to animate openings with outdated attire that the Strawhats likely will never canonly wear again, especially if the whole purpose of the opening is to attract a new audience.
Well this is nothing new. This happened frequently back in the first half. The "default" outfits are about as iconic to the second half of One Piece as the We Are outfits were to the first half. And just like in the first half, the openings will eventually stop focusing on them, but not for a while. And really, we're yet to see how often Luffy actually switches outfits, same for the other Straw Hats other than the ladies who are always going to change for fanservice purposes. Keep in mind that the only reason Luffy and the guys are wearing the costumes they are is because they were forced to by Law when they first infiltrated Dressrosa. The anime producers are likely waiting to see if Oda gives them "true" new outfits in the next arc or if they return to their "default time skip" outfits.
4. Colour Spreads
Oda has made some really beautiful colour spreads over the years that would be amazing if brought to life in an opening. Oda has made wayyyy better "group photos" of the Strawhats than any of the ugly ones in the last 4 Openings.
They frequently use color spreads. Hands Up, as far as I know, used the majority of the color spreads that Oda drew for the Fishman Island arc. I think they've been letting them pile up a bit and are probably saving them to use for the next opening once Dressrosa is finished in the anime. There will be quite a few to choose from by then.
5. More focused theme and direction
How many characters were thrown at us randomly in the newest opening? Someone should count, because it felt like over a hundred. Too many characters and not enough focus. Opening 13 was an Ace tribute theme, and Opening 14 was a 3d2y theme. But the last 4 openings have been a bunch of scenes thrown together without theme or direction.
I expected more of a Sabo theme out of Hard Knock Days, but ironically enough they gave Luffy more flare with fire in the opening than Sabo and Ace.
I think this comes down to more personal taste and subjective opinion. I can definitely see what you mean and probably agree to a certain degree. But at the same time, at least some of the scenes really worked for me along with the music timing. Things, especially with Toei, could definitely use improvement, but I think there have been worse structured openings and endings in the past.
1. Short glances at current/past islands.
Just imagine how cool would it be to have aerial views of some of the old islands, especially some of the more aesthetically pleasing ones like Water 7, Skypeia and Enies Lobby in HD. Most of the openings use sedating music and visuals, so this idea would really fit in. And no, I don't consider Japan a canon Island to the One Piece universe.
3. Group photo of Strawhats as kids, then morph to pre-timeskip designs, then morph into post-timeskip designs
I've always liked this idea, and I'm not sure why it hasn't been implemented yet. It works out so well because all the crew have had flashbacks to when they were kids (in the case of Brook, having flesh), then they also all change post-timeskip.
Only reason why I can think of why it wouldn't be done is because of Brook losing his flesh. Might be a bit too morbid for One Piece…
For these two, I like your ideas and wouldn't mind seeing them implented. It would definitely add something a bit different. For Brook, they could potentially get away with just using a sort of effect without going insane with showing flesh rip off, like use sparkles or transition scenes using a slash of his sword or something.
For all you know "Japan" could be a legal One Piece island but we'll have to see. We have "Wado" which hints at being heavily Japanese based. We also have the "Happo Navy" which from what I've heard is hinted to be more Chinese based.