Learn Finnish: Arvokkuus on yhä harvinaisempi piirre. - Dignity is a more and more rare feature.

On its own, harvinaisempi just means 'rarer'. When you add yhä in front of a comparative, it indicates continuous change, translating to 'more and more rare' or 'increasingly rare'. If yhä were used with the basic adjective harvinainen, it would simply mean 'still rare'.

I recognize the root of arvokkuus. How exactly is this abstract noun built?

It is a layered derivation. It starts with the noun arvo (value). Adding the suffix -kas creates the adjective arvokas (valuable or dignified). Then, the -uus suffix turns it into the abstract noun arvokkuus (dignity). Notice the consonant gradation: the single k in arvokas becomes a double kk before the -uus ending.

What is the specific rule for forming the comparative harvinaisempi from harvinainen?

For adjectives ending in -nen, you must replace the -nen ending with -se- before adding the comparative suffix -mpi. Therefore, harvinainen becomes the stem harvinaise- followed by -mpi.

If I wanted to say 'Dignity is BECOMING a more and more rare feature', how would this sentence change?