I would love the ability to add multiple categories to any one (1) Planned Spending expense under the Spending Plan...similar to the way you can do it with the Watchlist.
For example, I have these category groupings and each one has multiple subcategories.

I'd like to tie the top two (2) to this one (1) Planned Spending expense.

Personally, I like grouping my Categories together to give me a little more granularity when running reports, etc.