Back in July, Marvel confirmed at Comic-Con that the villains in Captain Marvel will be the iconic aliens known as the Skrulls, and that the movie will be set in the 1990s. While the cast is starting to be built around Brie Larson's Carol Danvers, with Jude Law portraying Mar-Vell and Ben Mendelsohn portraying the villain, rumored to be Yon-Rogg, new details have come forth about three important characters that are currently being cast, which may all be key Skrulls characters. There will be potential SPOILERS below, so read on at your own risk.
The brief casting breakdowns do not specifically state that these characters are in fact Skrulls, but there is some evidence that these mysterious characters named "Alpha," "Delta," and "Supporting Male #2" may in fact be Skrulls known as Annelle, Dorrek VII and Titannus, respectively. There has been no confirmation as of yet if any of these characters will be featured in this story, but it was previously rumored that Mar-Vell would be part of this story, and just a few months later, Jude Law was cast as this character, so it wouldn't be surprising if this rumor is true. This story is also set to follow the Kree-Skrull war as well, so it remains to be seen how many prominent Skrulls will be cast.
This report claims they are "very confident" that the "Supporting Male #2" character is in fact Titannus, with the production looking for actors between 35 and 45 for this role. The character is described as, "a chiseled, cocky guy who is large in stature, but not as big as Delta," which admittedly isn't much to go on. In the comics, Titannus went through the Super Skrull program which gave him incredible strength and healing abilities, but also an uncontrollable rage that lead him to flee to Earth, where he was involved in a major battle with the Avengers in Japan. What's interesting is there have been reports that part of Avengers 4 is shot in Japan, and that Avengers 4 will feature Captain Marvel returning after a long absence. This report also speculates that Titannus was seen in the concept art unveiled at San Diego Comic-Con, but that was never confirmed.
As for Alpha, that character is speculated to be Anelle, the daughter of Skrull Emperor Dorrek VII, with the Delta character rumored to be Dorrek VII. In the Marvel Comics, Dorrek VII was the Emperor during the Kree-Skrull war, during which Anelle fell in love with the Kree hero Mar-Vell, who we now know will be played by Jude Law, so that could add an interesting dynamic to this story. They had a forbidden love child in the comics, who is sent to Earth, where he was raised as Teddy Altman and would eventually become a young Avenger known as Hulkling. The studio is presumably casting a rather wide net for this role, though, seeking actresses between the ages of 20 and 49 who have fight experience, with "dance or gymnastics preferred."
Marvel is seeking males between 20 and 49 for the Delta character, who must be over 6 feet tall, "broad shouldered and muscular." If this character is Dorrek VII, he could be one of the main villains along with Yon-Rogg, but as of now, nothing has been confirmed. Anna Boden and Ryan Fleck (Mississippi Grind) are directing from a script that is was originally written by Nicole Perlman (Guardians of the Galaxy) and Meg LeFauve (Inside Out), and is currently being rewritten by Geneva Robertson-Dworet (Tomb Raider). This unconfirmed report first surfaced from That Hashtag Show earlier today.