I've seen we have a tag for bleach, naruto, dragonball-z, etc. Should that be what we do going forward for every series? Have a neon-genesis-evangelion tag? A fullmetal-alchemist tag? Or will that give us way too many tags, since there are quite a few different series out there? Is there some threshold of popularity you need to merit a tag for a given series?
Yes. A series tag is a vital part of a question on a site about literature, gaming, et cetera, and are a great example of the type of tags a site should have.
Represent an area of expertise: It's possible for users to be an expert in a specific series, having an in-depth knowledge of it's world and characters.
Useful as a Filter: Ideally our site will have a large volume of questions. With this in mind being able to easily sort by series is essential to make finding relevant questions easy and painless. Can you imagine trying to track down a question for a specific series if we didn't have series tags? Not only would it a painful experience, it would be enough for people to not want to use the site (I wouldn't. Searching would be impossible)
A Tag People will Follow: Users will always be willing to follow (favourite) a series tag, and are likely to be among the top followed tags on the site. Due to this, they should exist.
A Tag People Will Want to Ignore: If I don't read a series, then I won't care about any of it's questions aside from moderation purposes. Due to this I should be able to ignore any questions from this tag so they don't clog up on my front page. This is also very useful for spoilers. If I'm currently in the process of watching a series which I'm not up to date on I won't want to stumble across spoilers when I visit the front page of the site. The ability to ignore and hide tags is ideal for solving this problem.
There's no other way to identify which Anime is applicable to the question other than in the question title. This can get unwieldy - especially for longer titles.
And also for questions that apply to multiple Anime series such as: "To Aru Majutsu no Index" and "To Aru Kagaku no Railgun".
It's better to just apply both tags instead of trying to squeeze them into the title.
I suggest you consider not just holing yourselves into the "one tag per anime/manga" mindset for a number of reasons we learned the hard way in Gaming:
- Tag length limit. You'd be surprised how often this comes up. Want to talk about "Assassin's Creed 3: Liberation"? "assassins-creed-3-liberation" is way too long, "assassins-creed-liberation" is 26 characters...
- Title localization. Are you going to use "death-note" or "デスノート"? You can accept both, but you need to pick the one that goes in the titlebar — and that's very important Google-wise. So you might decide "okay, let's go with the English title whenever possible because the site is in English." What about animes that lack an official title in English, then?
- Lack of character support. Remember what I said about accepting "デスノート" as a tag or at least a tag synonym? Yeah, you can't do that. Yet. UTF-8 support in tags is currently only being tried on Japanese.SE, afaik. Sure, it's easy to see a request for enabling this feature here too granted, but you should still be aware that it comes to the price of stability.
- untagged. A work that doesn't get two questions asked about it in the same six months gets retroactively changed into untagged. If you rely solely on tags to know what anime/manga you're talking about, that... is very bad news. Thanks to Patents.SE and much pressure, Arqade managed to disable this process at the cost of, well, automatically pruning tags that do deserve pruning. That's another "special needs" request you might want to look into, at any rate.
Additionally there are a few ways that would make this even worse for Anime.SE than it is for Arqade:
Questions that are not about a single manga are on-topic. On Arqade questions about games in general are regarded as "game design questions" more appropriate for GameDev. This is not true here and there's already plenty of questions about anime design, reading and production in general. What I've found is that "one tag per game" tends to cannibalize all other forms of tagging (consider for example plot-disconnect, which one could easily consider a poisonous meta-tag that isn't useful as a filter or as an area of experitse.) The thought process that kicks in eventually is that "single work tags" are the superior kind of tag and everything else is useless. You clearly don't want this.
Even when it comes to "single work per tag," you still need to distinguish between anime and manga versions. Except when you don't. Arqade approaches this by using (for example) "mass-effect-series" when asking questions about all Mass Effect canon (as opposed to "mass-effect" which is just about the first game), but such an approach is less likely to work well here. Consider "one-note": is it about the manga, the anime, both?
It is silly to say "don't make tags for single anime/mangas" (you will want tags for all major works - but where do you draw the line?), but going the opposite side and making a tag for all single animes or mangas brings about a lot of challenges, most of which aren't actually fun or rewarding to overcome. You end up wasting time on how to tag "Asaoka High School Baseball Club Diary: Over Fence", cursing about untagged and generally not spending time actually doing questions and answers.
This is still day one and you still have a chance to break from the tyranny of "one game per tag." I suggest you consider it, starting perhaps by being precise in either the question title or body about what your question is actually about.
I think we should take a cue from Science Fiction and Fantasy here; for a distinct work, have a tag for the work. It doesn't matter if there's only one question for the tag, if it's about a legitimate on topic work, then the tag should exist for searchability as well as future questions.
If the work is part of a series, then a tag for the series can exist as well.
If this approach is taken, then we should request that the script that deletes tags that have a low number of posts on them be disabled for this site.
Thinking about this more, we might actually need more than one tag per series. Given that each series has commonly accepted Japanese and English titles, we'll probably need to be able to tag the question with both of them.
I don't know that synonyms are a good thing here, because it might cause flame wars over what the preference site-wide should be (and we certainly don't not have the space for two tags for a series on the question).
Honestly, I'm not sure if there is a good justification for any other kind of tag, at the moment.
The first tag tends to dominate SEO considerations (since it is prepended to the page title), and should generally match the series name.