Sleep debuff debuted with Summer Abigail, and Bunnytoria can shuffle cards, something that otherwise only a single mystic code in the game can do. I may be wrong, but i think Summer Martha was the first servant to ever care about terrain. With 3 SSR units, you would think they could cook up something that wasn't done before, or at least iterate on something relatively fresh, like star count matters(Jane does it, so they certainly have the technology), or field change like Arc, or anything else really.
Edit: Turns out the first servant that cared about terrain was actually Gawain. In my defense, the release of Camelot and Summer 1 are separated in the span of weeks. Also, back them it was still considered an original gimmick.