ORAS rebuilds Hoenn with Mega Evolution and Primal Reversion on top of the original Gen 3 evolution methods. Twenty new Mega forms join the XY set, and Primal Groudon and Primal Kyogre are effectively permanent Mega Evolutions with 770 BST. DexNav helps find evolution-chain starters with hidden abilities in early routes.
Twenty new Mega forms debuted: Mega Sceptile, Mega Swampert, Mega Sableye, Mega Sharpedo, Mega Camerupt, Mega Altaria, Mega Glalie, Mega Salamence, Mega Metagross, Mega Gallade, Mega Lopunny, Mega Slowbro, Mega Audino, Mega Steelix, Mega Pidgeot, Mega Beedrill, Mega Diancie, Mega Rayquaza, and Primal reversions for Groudon and Kyogre.
Primal Groudon and Primal Kyogre automatically revert to their primal forms when they enter battle while holding the Red Orb or Blue Orb. Unlike Mega Evolution, it doesn't take your Mega slot. You can have both a Primal and a Mega on the same team. Primal forms hit 770 base stat total, making them the strongest non-Mega Rayquaza Pokemon in the game.
Two methods now. The classic Gen 3 way still works: max out Feebas's Beauty stat with Pokeblocks and level it up. ORAS also added the Gen 4 method: trade Feebas while holding a Prism Scale. The Prism Scale is easier to find than making perfect Pokeblocks, so most players go the trade route.
Indirectly. DexNav shows hidden Pokemon on each route and can find ones with egg moves, hidden abilities, or higher levels than normal. This means you can find evolution-chain starters earlier or with better setups. It doesn't change how evolution works, but it makes getting the right starting Pokemon much smoother.
Yes. Rayquaza is the only Pokemon that Mega Evolves by knowing the move Dragon Ascent instead of holding a stone. This means it can hold any item while Mega Evolved, which is a huge competitive advantage. It was so strong that competitive formats created a separate "Anything Goes" tier specifically because of Mega Rayquaza.