Not casting those who have already been cast and even though it hasn't been confirmed, I'm assuming Evangeline Lily will play Wasp.
Alice Eve as Carol Danvers/Ms Marvel
An under appreciated actress who's acting abilities gets jilted due to her looks (see Star Trek: Into Darkness).I find her acting very credible and she has the ability to portray someone who can without a doubt give the toughest hero a run for their money. Alice is my choice because she can deliver her lines in a way that you won't be focused on her beauty the whole time and actually accept her as the popular superhero. Her origins may have to be glossed over in this movie but they said she's already been eyed for her own film so I wouldn't worry about that.
Ryan Gosling as Captain Marvel
Casting Captain Marvel was tough for me so I narrowed it down to two choices and this is assuming Nathan Fillion won't be portraying him (which is unlikely but anything is possible). Ryan Gosling is an underappreciated actor that could bring the badass to Captain Marvel in a cameo or an after-credits scene because this still has to be Ant-Man's movie. Adding in Gosling brings the star-power to the film without overshadowing the main plot and characters. I would like to think Gosling would be okay with a simple cameo until Ms. Marvel's film but if not I think my next pick would be cool with it.
Ben Foster as Captain Marvel
Another underappreciated actor I feel could play the hero. If Gosling or Foster were to get the role I would personally want a Captain Marvel film before a Ms. Marvel film but if Marvel (too many Marvels) doesnt want to go that direction then I could see Ben Foster playing the role whether the character appears in Ant-Man, GOTG 2, or whatever. I doubt this character would appear during the next phase but anything is possible.
Dave Franco as Alex Power/Powerpax
A very promising leading man who can without a doubt contribute to the comedy that Ant-Man is shaping up to be. Franco can find his place in Hollywood by portraying Alex Power, a hero who can control his own mass and disintegrate at will. Powerpax is a unique character that I feel should be brought to the big screen in Ant-Man.
Willow Shields as Cassie Lang
The daughter of the main character I believe Willow could play a good Cassie Lang and (if Marvel decides to go in that direction) a good Stature.
Olga Kurylenko as Agent Anesthesia
Olga is a good actress that can play Agent Anesthesia, the secondary villain in the film who will serve as the personal quarrel for Lang, considering she works for AIM only for help for her father who is in a coma.
Emma Watson as Jeanne Foucault/Finesse
A polymath with the ability to instantly master any action by looking at it once. It is suspected that she might be the daughter of the Taskmaster. Emma Watson is a proven actress that, in my opinion, is looking for that next breakout role in her career. Playing and popularizing the character if Finesse will not only offer that but offer some star power to the Ant-Man film itself.
Marion Cotillard as Jennifer Walters
An Easter egg for fans of She-Hulk. Although the green gal won't appear in her most-known form, the human version of her, played by Marion Cotillard will appear. Marion is considered by some to be one of the greatest actresses of all time and I feel she could bring Jennifer Walters to life the same way Ruffalo brought Banner to life. You should be interested in the character before they turn into giant green savages. I feel Cotillard could bring that to the role better than the normal picks like Ronda Rousey (why?) and Gina Carano.
Jayma Mays as Rita DeMara/Yellow Jacket
Jayma is a solid actress and considering this film is taking place after Hank Pym's run as Ant-Man then they should have Rita DeMara, who stole his identity at one point and is known as the villain Yellow Jacket.
Dustin Clare as Taskmaster
With work on shows like Spartacus he certainly has the physicality to portray this villain and his acting chops have been proven to be more than okay. Should Marvel choose not to go full-on CGI then Dustin Clare would be my pick for the man villain of Ant-Man.