Helioptile evolves into Dracyrus lv. 32.
Bunnelby evolves into Burybun (Normal/Ground) lv. 22.
Swirlix evolves into Sweefloss via Happines.
Spritzee evolves into Fraganzee via Happiness.
Inkay evolves into Malimar by holding upside down your Nintendo 3DS while leveling up (right after the battle is finished).
Excluding Eevee, no older Pokémon have new pre/evolutions (Mega Evolutions aren't real evolution).
Gym Leaders are Bug, Fairy, Electric, Rock, Fighting, Ghost, Dark and Ice. Elite Four are Poison, Ground, Water and Grass; you can battle them in any order you want. Champion is mixed.

In the third city you reach, a woman will give you an Eevee (one of your rivals also get an Eevee). She also introduces you to the mechanics of Pokémon Aime.

Old Pokémon are in the Kalos Dexes (there are three different Pokédex). Some examples are Pikachu, Teddiursa, Pidgey, Zigzagoon, Aipom, Riolu, Venonat, Staryu, Vanillite and every Pokémon that was retconned to Fairy-Type.