I'm (for example; it doesn't have to be this specific example) in the middle of building a deck, but I have one more slot open and was wondering what card would be good there (the options are fairly limited; you don't need to be very familiar with more than ~30 cards that I have as options). Would this be an acceptable place to ask what would work best given the criteria I'm already looking for?
Questions like this are on topic, but they may not be well received. One one hand, this question is what we call "good subjective"; you are asking for a recommendation with criteria specific enough that the relative pros and cons are enumerable and someone could write up a complete answer explaining why one choice or subset of choices is best. We generally like questions like that, and it would be on topic.
However, a question like the one you describe is unlikely to help any other visitors. Nobody is going to come along with the same partial decklist looking for the same last card. That might cause the question to attract downvotes. You can mitigate this by trying to make the question more generally useful, but it would be a balancing act between making it too broad ("What is a good blue card to add to my deck?") and too specific ("Here are 59 cards and n choices for the last card, which is the best choice?").
I recommend that you ask your question. The worst that can happen is that your question gets closed (so you don't get an answer) and you lose 2 or 3 percent of your rep to downvotes. And the rep loss would be cancelled out if the question gets even one upvote.
Let's start with one thing we can all agree one. We do not want this to happen:
- (Today) What is a good card recommendation for my mono red aggro deck in the current standard?
- (A year later) What is a good card recommendation for my mono red aggro deck in the current standard?
- (Two years later) What is a good card recommendation for my mono red aggro deck in the current standard?
These types of questions can be improved to a point where they meet my "good subjective" criteria. Others may disagree with me, but card recommendations just don't cross into good subjective territory. Process recommendations do. The question should ask for a process, not a card. Here is how I would transform the above example:
What is a good process for choosing cards that fit in a mono red aggro deck in standard? I am building around Card X and Card Y because of Reason 1 and Reason 2.
Sometimes you can't transform a question without stripping away the original intent of the author. If the author is hell-bent on simply getting card recommendations for his specific deck, close the question. Helping people is nice, but there are better outlets for this type of thing, such as TappedOut.net, a site dedicated to building specific decks and getting specific feedback.