Yes, that’s a nice workaround that would work well in some cases, but not so well in some other situations, for instance, when some (or most) of your notes have duplicated “card categories”. Let’s say that I have in my deck notes like this:
Mother Teresa was a humanitarian. She was born in Uskub, Ottoman Empire on August 26, 1910.
In 1932, she…
In 1950, she…
In 1962, she…
In 1992, she…
…and I’d like to create a filtered deck for {{c10:: … }} important_years, this would be my note:
{{c1::Mother Teresa}} was a humanitarian. She was born in {{c2::Uskub}}, Ottoman Empire on August 26, {{c3::1910}}.
In {{c10::1932}}, she…
In {{c10::1950}}, she…
In {{c10::1962}}, she…
In {{c10::1992}}, she…
…and of course this is not a well designed note under any point of view.
I think the best and simplest solution for this and other related problems is your idea of expanding the flag system, maybe allowing the user to create as many flags as he needs.