Use custom mirror axis

[JacobDoesLife]

Would it be possible to add an option to the mirror for a custom mirror axis?

[lbalazscs]

If you mean the Mirror filter, it already supports mirroring along an arbitrary axis.

If you're referring to the "Mirror" option in some of the tools, adding a UI for selecting a custom axis might introduce more disadvantages (e.g., making the overall UI more confusing) than benefits. However, in most cases, there's a workaround. For example, you can add a new layer, apply the standard mirror options, and then use the Move Tool or the Filter / Other / Transform Layer filter to reposition or transform it.