An actor named Trey Sartorius uploaded a picture on Instagram that was very intriguing a few weeks back.
Instagram post: Click Here
Trey isn't a well known actor, but plays a bit on Nickelodeon and is very young. He posted a picture of the Supergirl DEO set with the caption: I can’t wait for everyone to meet (Mon) Thank You @supergirlcw for giving me this amazing role alongside the legend @melissabenoist He posted this about four weeks ago, which is about the time the Supergirl cast finished filming. So, that means he is definitely near the end of the season and since we haven't seen him yet he will be appearing in 4x22, The Quest For Peace. The season finale. In the trailer, it has no mention or shots of him which means he is most likely one of the cliffhangers. Now, his name, Mon. I believe that is foreshadowing Mon-El, and if it is, it would tie everything together. Especially since it is confirmed that the League of Superheroes will be returning in season 5.
Now, for the season 5's big bad.
In episode 4x21, Red Dawn, Brainy and Nia were captured after an attempt to save aliens who have been wrongfully imprisoned. Due to Brainy's strong emotion towards Nia, he fought through the torture they put him through and in doing so, they damaged his brain mainframe. As you know, Brainy's ancestors and relatives are insane. Brainiac, who is an infamous Superman villain, and Indigo, who appeared in Supergirl season 1. He says that he has become the same as his ancestors. He has become cruel and lets Nia and J'onn be captured as a way to "save the aliens". This is an obvious problem that was easily overlooked in this episode.
And so, if you put all of these together, all signs point to Brainy being season 5's big bad, or closely related. This would explain the new character Mon, the reasoning for Brainy turning bad, and the League of Superheroes returning in season 5.
Update: Looking over Trey's page, I saw some more clues. He says in his IG story that his character is from Daxam, he is from the future, and he has come to save the world from an upcoming doom. Ominous, right? He also is not allowed to release his character's last name since it spoils a lot and that is very suspicious. I guess my theories on his character were correct! But, still nothing known about Brainy being potential big bad.
Please tell me if you have any other ideas and/or feedback! It's much appreciated!