What the time skip does is allow us to become uncertain of the existing cast and to see their off-screen development based on where they were. Aqualad finds out the villain Black Manta is his father AND his love interest Tula is killed, therefore when he shows up as a villain we aren't sure if he is or isn't.
The majority of the drama in season 2 is a Cold War situation where Nightwing, Aqualad, Artemis and Wally have made a secret pact that no one else knows about to go deep deep undercover. Up to the capture of teammates the killing of team members and the destruction of their base. There is a powerful moment about 10 episodes in where Wally breaks down in anger and terror about how he cannot be 100% sure that their ruse is real because from his perspective teammates are captured and bases are destroyed and his fiancee is so deep under cover that a good 3 episodes are dedicated to her balancing acts for and against the team for her mere survival.
Miss Martian has an entire season long story arch about her flagrant immoral use of her powers (which started in season 1 when she mind blasted Psimon and lied about it).
The best thing I ever read about Season 2 is that in the therapy ep of season 1 the team all tell Black Canary what they are most afraid of.
Aqualad is afraid that he is more of a soldier than a leader and his willingness to be on the front lines will get the team killed.
Robin was afraid he would become like Batman and ignore the feelings of his friends and do WHATEVER IT TOOK for the mission.
Wally was afraid of Artemis dying.
Miss M was afraid of her team discovering what she was and hating her for it.
Throughout season 2 all of those fears are put to the test. The team are dragged through the ringer.
In order to sell a season long story about deep cover, betrayal and doing whatever it took for the mission, they needed to bring in a new list of characters to up the tension, to replace the Justice League while they are on Rimbor and to ensure that trust was indeed tested.
Plus we get Impulse, Blue Beetle and Arsenal out of the mix and they are all fantastic.
Oh and Static is in season 2. I miss Virgil so much. If there is ANY reason I need season 3 it is for Virgil.