I recommend you to get to eternal ASAP. The longer you wait, the harder it becomes. As seen at the bottom of the arena menu, you unlock various legends and vanities. The new Back vanities are only available to those who reach Eternal rank The top ranking Eternal players will also be receiving a special reward at the end of the season. No deadline yet because we have yet to create the rewards.
One of the reasons why I am choosing not to join the public Season is so I do not take away from the limited rewards given out. I already have my vanities and everything unlocked so giving the chance to normal players is for the best. For now just have fun and try your best to get to Eternal before all the spots are taken. Once you are in Eternal, try to stay there or let your ranking fall for others to have a jap at the Vanities, your choice.
Also get to Magic League in both 1v1 and 2v2 to unlock the auras.