Rocket League will introduce a new tiered item unlock system this summer in which players earn cosmetics for leveling up. The "Rocket Pass" will have both free and paid-for paths. The free version will offer cosmetics—decals, wheels, toppers and the like—as well as decryptors that open crates. The premium pass will be unlocked for a one-off fee and, presumably, will offer better, more regular gear as you progress.
Each pass will last a "few months" before it's retired and you need a new one. Psyonix didn't specify whether the one-off fee will cover you for all premium passes, or whether you'll need to pay each time it expires, but it mentioned that it was "still developing" the idea, so look out for more information soon. It sounds an awful lot like the Battle Passes you can buy in Fortnite, which reward you with cosmetics as you play.
It detailed the new system as it laid out its road map for the next few months: this month, players are getting a new arena, next month they'll get a summer-themed event alongside new licensed DLC, and in July there will be a "big anniversary in-game event with some special throwback content" to mark its third birthday.
Arriving later in the year are cross-platform parties and a new leveling system that removes the level cap and addresses the ramp-up in XP that you need to unlock higher levels.
You can read all the details here.