Prerelease Primer
MaRo's article mentions that
* Explore came from design. It was the highest rated mechanic in internal polling, so it never got changed by development.
* Amonkhet originally had vehicles, with the intention of it becoming evergreen, but there was already enough going on in the set, and they realized this would be a consistent problem, so it was made deciduous and won't appear at common.
* Plunder was a bloodthirst variant, where on ETB, you looted if dealt combat damage. They kept on looking for a raid like mechanic, until they just used raid.
* For dinosaurs, they explored the space of monstrosity and the Phantom mechanic, but it didn't seem like a good fit.
* Merfolk focused on prowess, which did mean testing prowess in green. There were multiple issues, including prowess being weird in green, spells matter diverting from the tribal theme, and them feeling prowess is too complex in large numbers at common.
EDIT: BTW, if anyone is unfamiliar with any of those terms, feel free to ask.