I think that when you want to focus on the variety of the content a paysite has to offer, then you could try mixed pics:
big boobs with big nipples, big boobs with small nipples, ...
small asses, fat asses, ...
messy feet, manicured feet, ...
white stockings, yellow stockings with green bullets, ...
But on the other hand, this is also a bit tricky. You tell a focussed surfer that he only will like 20% of the content.
Myself I always use series and I have no plans to change that
