Activating Markdown Extra you can add special attributes to certain elements as headings or code blocks but not paragraphs.
Kramdown allows to do it using block attributes. Maybe is possible to add this option in a future version of Kirby.
You can swap out the Markdown component for your own 
https://getkirby.com/docs/developer-guide/plugins/components
That’s a good solution.
Anyway, include the option to add classes to paragraphs to Kirbytext would be great.
You might as well use custom Kirbytags…
Yes, that’s another custom solution.
I’ll check it out.
Thanks