Not sure if I'm doing this right, never made a meta post before.
So we already have flaired users, whose flair demonstrates their knowledge on a subject, but I think it would be a good idea to take it another step. Right now we have lots of interesting questions in some subjects, but they often get buried by influxes of equally interesting questions from other subjects.
If it were possible or practical, these questions could be flaired by the author to fall into a certain category. All questions of a certain subject can be filtered by category on a main page (somewhat similar to the filters on /r/worldnews) which will display only the questions of that subject.
Additionally, one can potentially subscribe to a particular subject where an automod can notify the subscriber if a new question has been posted (or more practically, give a list of all new posts in a certain length of time; perhaps anywhere from 3 hours to a day).
By doing this, people that come here looking for questions on a subject they like, either to wait for/read responses or answer themselves can find them extremely easily, and people that ask the questions in the first place have a better chance of getting answers as well.
How bou?
This has been suggested (and considered) in the past. The big problem is that submissions can have only one flair, and there's no good way to:
a) get a clear classification system
and
b) shoehorn questions into a single category.
But what about our user flair system? Frankly, it is a mess. We have flairs based on geography and based on subject, forcing users to choose one or the other. An expert on the Punic Wars might be flaired Punic Wars, Punic Wars, or even Punic Wars. There is no rhyme or reason to it, we leave it up to the user to decide. That's not a huge problem since users generally aren't searched for based on their flair category.
For questions on the other hand, it would be a problem. You say you want to use it to filter certain topics or subscribe to some. It would be a shame if you're interested in military history and missed out on an excellent thread on the Punic Wars because it was tagged Europe instead of Military. And this is a simply one example. What single category would the economic and military competition between the Dutch and the Portuguese in South East Asia fit in? Or the European slave trade between Africa and the Americas? And so far I have ignored the option of having a classification based on arbitrary time periods! There are so many options that it's not feasible to make it work with just a single flair per submission.
Note there is a way though to get sort-of 'subscriptions': we suggest using IFTTT - as explained here.