I have another suggestion that builds upon this:
dummy CSS markup classes with custom delimiters.
I really like and use the custom CSS feature! 
My proposal is this: there are several (maybe 3-6) dummy CSS markup classes without pre-defined delimiters; like a template. When a user sets delimiters in the settings, those do nothing at first (just inherit markup; or have no markup at all). But then the user can use custom CSS to define the markup they wish.
This would allow us to
a) have custom text highlighting
b) be able to grey-out text (similar to check but without strike-through)
c) have text underlining as suggested here: Add ++Highlighter++ formatting
d) use other CSS markup
e) have text segment colors and background colors as suggested here: Format text font and highlight with colors