Does there need to be a royal link? You don't explicitly SAY so - your question comes across to me as quite 'loose', not very specific.........but everyone knows that GK married into royalty. It sounds like your teacher simply wants you to 'compare and contrast' EFFECTIVELY, so you could pick just about any 21C celeb, really - just talk about what similarities there are in their lives, then what differences there are.
You could even choose a male (difference (or contrast)), not even in the acting field (contrast, again). It could be someone who has (similarity) or hadn't (difference) give up their career for something else (e.g. - Amy Winehouses career has virtually given HER up!!, because of her addiction; the contrast here is that GK gave her quite successful career up for marriage into royalty (of another country)). The 'compare/contrast' goes on - countries of birth, socio/eco backgrounds, political leanings, marriage(s), even colour of hair, favourite colours!!, (if you want to get down to the ridiculously insignificant!)
I guess if you were looking for someone royal, just find someone (anyone!) who is a 21C celebrity who is SOMEHOW linked with royalty. Catherine Oxemburg??
And then expand all the similaities/differences between them to make your full report.
Confused? .... so am I (LOL)