Since there are many games that can be played by one person, I feel it would be useful to have a related tag.
No, I think this is a Meta-Tag:
The reason meta-tags are a problem is that they do not describe the content of the question.
We should be careful when tagging using one of those tags. It should only apply when the question is about a game that could be played in solo and the question is about that. (a question about Spider (Solitaire) should not have that tag).