Those half second flashes they show of upcoming material or characters are only spoilers when you have CONTEXT for them. Otherwise they're just weird crazy flashes of action, that mean as little as the cast re-enacting random color spreads or Mihawk and Shanks appearing in every single opening. Especially in shows that create made up action sequences for their openings that represent things that are coming… but don't quite happen that way, or shows where the anime strays far from the source or is the original content.
For those that are already aware those bits are coming, its not spoiling anything, its just a tease. For those that don't, its a contextless image of... something. I don't know how many anime series I've watched where the opening plays the entire season, and its not until episode 23 that something happens, and suddenly that moment in the opening I've seen 23 times with no meaning actually has significance and the opening suddnely has enhanced context and meaning.
If you already know, its not a spoiler. If you don't know, its not a spoiler. If someone complains in front of you "No, that's a spoiler!" and you didn't know, then its only them telling you such that it gives it away.
The only real exception to that is when they blatantly placed Robin in as part of the crew before Alabasta was done, and even at the time I questioned "What is SHE doing in there?" and was curious about what the hell was coming up... and they've been far better about not giving away big things like that since. (They went well out of their way to avoid giving away Franky and Brook as crewmebers.)
AND EVEN ALL THAT ASIDE, where OP is concerned... It's kind of, uh, massive in Japan. Inescapable. Not being aware of coming is more of a deal stateside maybe, but in Japan?
By the time these hints appear in opening animation sequences, the references are already common knowledge anyway. Millions of Japanese viewers have already read the originating manga stories well beyond the events depicted in the anime. It’s also difficult for Japanese viewers to avoid being exposed to hints about the future of popular anime series through advertising and merchandising.
Small revelations and brief glimpses of things to come revealed in opening animation sequences may be more accurately described as teases. These brief hints whet the appetite of viewers, teasing them with the promise of what’s to come. For many Japanese viewers, these hints aren’t spoilers because many Japanese viewers already know what shonen and shoujo adaptations of adventure manga have planned. These brief shots of future plot developments also please fans in retrospect because after the references come to pass, viewers can reflect on these opening animation sequences and recognize all of the clues and “spoilers.”
I won’t deny that anime opening sequences sometimes include “spoiler” references to characters and events prior to them appearing in the anime, but these references are usually brief or partially obscured, and they typically don’t reveal characterizations, nuances, and the situations surrounding these glimpses of the future. The value of these “spoilers” to create anticipation for future episodes and create exciting opening animation sequences, I think, outweighs the relatively minor compromise in storytelling suspense that they create.
Ed and Al might fight chimera things! (They do, but never in the way shown.)
That purple haired guy and the green haired girl in the first episode are important apparently! And the entire main cast will show up sometime in the next 26 episodes! And some red haired guy shows up eventually!
Oh, cool warrior symbolism there. What do you mean Ichigo actually visits a field of swords eventually? I had no idea till you told me!
Huh, I guess there will be more than one gundam pilot. I guess they did start the first episode by saying there were five "asteroids" and most of them have shown up by the third episode…