[The following includes potential SPOILERS for Supergirl season 2.]
Supergirl was confident enough of a second season (which it got, along with a move to The CW) that it opted to end on a cliffhanger. Kara and company's season 1 finale victory celebration was interrupted by a spaceship crash that turned out to be a Kryptonian-style pod near-identical to the ones that brought her and Superman to Earth as children.
We got to see that Kara herself was surprised by the pod's contents, but the screen then cut to black before we were allowed to know who or what she saw. Now we know which actor will be found inside the pod when season 2 picks up: Chris Wood.
Wood (The Vampire Diaries) had previously been cast for an unknown role in Supergirl season 2, but at the show's San Diego Comic-Con 2016 panel it was confirmed that his as-yet unknown character is indeed in the one in the Kryptonian pod in the season 1 finale.
Bleeding Cool recently reported that, because the pod was deliberately set to resemble Supergirl's and be discovered by her, that the character will most-likely be a DC Comics character named Mon-El. The character originated as a Superman-like visitor from space in the pages of Superboy and ranked pretty high on Screen Rant's own list of characters who might turn up in Supergirl season 2 - pod-occupant or not.
Also known as "Valor," Mon-El is a Daxamite, an alien whose powers manifest on Earth as near-identical to a Kryptonian's but with a vulnerability to lead instead of Kryptonite. Traditionally depicted as a young man who often wears a palette-swapped version of the traditional Superman suit, he's associated with The Legion of Superheroes - one of whose flight-rings has already been seen in an episode of Supergirl displayed in the Fortress of Solitude.
Of course, since Wood's character has yet to be officially announced, it remains to be seen whether he will be playing Mon-El. Certainly, with the confirmation that his character is in the mysterious Kryptonian pod, it seems unlikely Wood's role will be announced ahead of the Supergirl season 2 premiere - instead, the show may leave it to be a surprise for fans during the premiere episode.
The Flash season 3 will premiere Tuesday October 4th at 8pm on The CW, Arrow season 5 will premiere in the same timeslot on Wednesday October 5th, Supergirl season 2 on Monday October 10th, and Legends of Tomorrow season 2 on Thursday October 13th.
Source: Bleeding Cool