I suppose I should think it depends upon what way you think they are homogeneous. Do you feel they are similar in personality or appearance? Both? Are they usually the same ethnicity or gender? Do they come from similar social backgrounds? Are all of their names just variations on the name Jeffrey?

If you know what element is feeling stale, I would think the easiest thing to do is just to flip it. If they all are all young white women, make a middle aged, Guatemalan man. Even those superficial things can lead to drastically different characters. The experiences of growing up and aging in Guatemala would certainly lead to differences in worldview.

Are all your characters existentialists? Then give them a decidedly contrary philosophical bent. Make them devoutly Hindu.

Or just make your character a severe burn victim with horrible scarring over 97% of their body. Or make them have had an ovariectomy.

I've often found it useful to try and create characters whose disposition or background is far removed or antithetical to my own as both an exercise in my ability to write from a vastly different perspective as well as creating a more diverse stable of character and story ideas.

I suppose I'm pontificating now. I concur with Prisk that TV tropes is a fun resource when thinking about ones own narrative intentions.