So I think it's proved enough now: Every new expansion will have a set name that basically means "meet these new and exciting Pokémon that you have not met before, because they've been hiding until this very point". These new and exiciting Pokémon are mysterious treasures, they're secret wonders, they're great encounters, they're hidden legends, they're unseen forces!
The expansions apparently always have to be called the same, with some slight nuances - If the next set really contains Darkrai, Palkia, Dialga and Garchomp lv X though, Great Encounters is a great nuance of the name. Finally encountering all those lv X will sure be great! =D