I wish it were possible to have mandatory tags, like in meta.
In meta, you must specify one of the dark tags (e.g. discussion) to be able to post it at all, unlike the lighter "normal tags" (e.g. user-interface).
If we could have all system tags (I guess we'd need to include tags that are used in place of specific system tags in some cases, such as dungeons-and-dragons, as well as system-agnostic for when a system tag really doesn't apply) marked as dark mandatory tags, that would be great, but I doubt such a thing is possible on the main site (or at least, isn't something we can customise, and requesting such a feature would likely prove to be a waste of our time, because it won't happen).
Related declined old meta from Arqade on making a set of tags special (thanks @Someone_Evil).
As an alternative suggestion, I know there are warnings that appear when you have a certain tag selected (e.g. the dungeons-and-dragons tag has this warning). Is it possible to have such a warning when no tags are selected, or when a subset of tags are not present?
That way, although it would be somewhat overkill, we could have a rule such that a warning is shown unless you have a system tag (this does mean it would need to be updated every time someone created a new tag for a system, but on the other hand, it is just a warning, meaning a question can still be posted without "obeying" this "rule").
At the moment, such a warning might (occasionally) make a new user stop before using the dungeons-and-dragons tag, but if they just don't pick anything, no warnings appear, so no need to stop and consider. So long as they slap any old tag in there, they can post. If there's a "negative rule" in place, at least the warning is there straight away.
Unfortunately, it looks like this isn't possible either, judging by the response @Someone_Evil got from @JNat♦:
I'm a bit unclear what the limits of "regex based" tag warnings are. Of specific interest: would it be possible to show a specific warning if the question isn't tagged with one of a set of tags (or include phrases in the body)? (I'm thinking of system tags for RPG.SE, which are often missing from new askers.) – Someone_Evil Mar 8 at 1:00
Guidance like the example you're suggesting, @Someone_Evil, should go in the "how to tag" dialogue, shown in screenshots 2 and 3. Checking for the absence of tags isn't, as far as I'm aware, something we do anywhere (other than Meta sites). – JNat♦ yesterday
So, I've got nothing. I don't think we can ever solve this problem with the current Ask A Question screen and the customisation limitations we have. New users will always just not read the various notices we put there and will post whatever they want without tagging it properly, because they won't know (unless they read all the various help boxes that are within our customisation) that we require them to specify a system tag (for questions where one is relevant, e.g. rules clarification questions, etc).