user88: I've been asking in beta about anime crossovers 'cause I'm really interested about it. But they always vote for my question to be closed. Is topic about anime crossovers too off-topic to ask here? Just making sure so that my effort of asking won't be forsaken. Thanks a lot. 

user111: Your question is, and is not a list question at the same time. 
On one hand, it indeed needs a short answer containing only the anime name. However, there is one problem: we can't know everything.
What does that mean in your context? That means that the question will become a list question eventually. Some person might think "Hey ok, Anime AAA was the first one, since it was released in 1990", and post it as an answer. Then another person comes and thinks "Hey, but there was also Anime BBB which appeared a year earlier!", and posts this as his own answer. Then another person comes... and so on and so forth. So eventually it will be a list-competition question with people trying to remember earlier and earlier examples of crossovers.
Probably if you rephrase the question to somehow avoid such a situation, it will be gladly reopened.

user17: Pulling the comments into here:
When I first read the title:

What particular anime(s) first showcased crossovers?

It immediately struck me as being the typical non-constructive question. Why? Because it sounds like it is asking for a list.
At the very core, the question is fairly legit. There can only be one Anime that had the first crossover. So it has an objective answer to it. I won't go into whether or not that answer is obtainable as SingerOfTheFall's answer covers that quite well.
However, the trick to these borderline questions is that they need to be phrased correctly.
Depending on a how a question is phrased, it can lead to different types of answers. Phrasing it incorrectly could send the question into a downward spiral towards non-constructive land.
As your current title stands, the plural on "anime(s)" could possibly bait many answers from different people who have opinions on what is "among the first". But phrasing it as:

"What Anime first showcased crossovers?"

would tip it back towards that constructive direction. To make the question more constructive, you can request that all answers have authoritative evidence that they are the first. (Skeptics has a policy of requiring references to avoid this speculation trap.)
But as SingerOfTheFall mentions, it may be difficult in this case. But that's a separate issue.

user15: It's not really a list question. To me it's hard but reasonably narrow. A list question is "How many anime have a prick as a main character?" (lol just an example).
When answers start to go over 10, then you must worry.
In any case, it depends. As a moderator I've left open questions that could have been Not constructive but that only received one answer. So my opinion is: keep an eye on such questions, if too many answers start to come in, then vote to close.

88: <p>I've been asking in beta about anime crossovers 'cause I'm really interested about it. But they always vote for my question to be closed. Is topic about anime crossovers too off-topic to ask here? Just making sure so that my effort of asking won't be forsaken. Thanks a lot. </p>
 111: <p>Your question is, and is not a list question at the same time. </p>

<p>On one hand, it indeed needs a short answer containing only the anime name. However, there is one problem: we can't know everything.</p>

<p>What does that mean in your context? That means that the question will become a list question eventually. Some person might think "Hey ok, Anime AAA was the first one, since it was released in 1990", and post it as an answer. Then another person comes and thinks "Hey, but there was also Anime BBB which appeared a year earlier!", and posts <em>this</em> as his own answer. Then another person comes... and so on and so forth. So eventually it will be a list-competition question with people trying to remember earlier and earlier examples of crossovers.</p>

<p>Probably if you rephrase the question to somehow avoid such a situation, it will be gladly reopened.</p>
 17: <p>Pulling the comments into here:</p>

<p>When I first read the title:</p>

<blockquote>
  <p>What particular anime(s) first showcased crossovers?</p>
</blockquote>

<p>It immediately struck me as being the typical non-constructive question. Why? Because it sounds like it is asking for a list.</p>

<p>At the very core, the question is fairly legit. There can only be <em>one</em> Anime that had the <em>first</em> crossover. So it has an objective answer to it. I won't go into whether or not that answer is obtainable as SingerOfTheFall's answer covers that quite well.</p>

<p>However, the trick to these borderline questions is that they need to be phrased correctly.
Depending on a how a question is phrased, it can lead to different types of answers. Phrasing it incorrectly could send the question into a downward spiral towards non-constructive land.</p>

<p>As your current title stands, the plural on "anime(s)" could possibly bait many answers from different people who have opinions on what is "among the first". But phrasing it as:</p>

<blockquote>
  <p>"What Anime first showcased crossovers?"</p>
</blockquote>

<p>would tip it back towards that constructive direction. To make the question more constructive, you can request that all answers have authoritative evidence that they are the first. (<a href="https://skeptics.stackexchange.com/">Skeptics</a> has a policy of requiring references to avoid this speculation trap.)</p>

<p>But as SingerOfTheFall mentions, it may be difficult in this case. But that's a separate issue.</p>
 15: <p>It's not really a list question. To me it's hard but reasonably narrow. A list question is "How many anime have a prick as a main character?" (lol just an example).</p>

<p>When answers start to go over 10, then you must worry.</p>

<p>In any case, it depends. As a moderator I've left open questions that could have been Not constructive but that only received one answer. So my opinion is: keep an eye on such questions, if too many answers start to come in, then vote to close.</p>
