Recently a picture of a loot crate for Rocket League was leaked, and it has now been confirmed by Psyonix's Jeremy Dunham. The vice president revealed on Reddit that they do intend on adding loot crates to the game, similar to the mechanic used in Counter-Strike: Global Offensive.
Players will get access to crates full of random cosmetic items for their vehicles, but they can only unlock those crates with real money. Psyonix will not be using Steam's Marketplace for this, specifically to avoid third-party gambling sites exploiting their game, like has happen recently with CS:GO. Also, any player that wishes not to be a part of this, can hide all content related to the loot crates by checking a box in the options.
Psyonix also assures they will be continuing to introduce free arenas, modes, and items to the game, along with the occasional paid DLC. The full purpose of the loot crates, according to Psyonix, is to fund esports prize pools and events. More details will be coming at next month's RLCS Live Finals.