This thread on the Community Forum is what prompted me to make this wish.
I'd like to create conditional blocks based on whether a page exists or not. This is similar to a couple of older wishes, but this is more specific. I would propose syntax like this:
[[ifpage +category:page-name]]
The page exists.
Process this block.
[[/ifpage]]
[[ifpage -category:page-name]].
The page DOES NOT exist.
Process this block.
[[/ifpage]]
This could possibly be accomplished by wish:449 (over a year old) that proposes testing for page length or more generally by wish:102 (over two years old) asking for a broader scope of "if" tests. Neither of these old wishes have been accepted or rejected, so I'm not sure of their status.