Ant-Man was a surprisingly good film, and I believe it definitely deserves a sequel, which we will probably see in Phase 4 of the Marvel Cinematic Universe. That's a long way from now, but that does not keep fans like me from taking jabs at what we could expect in the sequel.
1. Search for Janet van Dyne
In Ant-man, we are introduced to Hope van Dyne who is the daughter of Hank Pym, and we clearly see there is tension between the two one of the reasons being the circumstances of the death of Hope's mother. Hank told Hope that her mother dies in a plane crash, but she doesn't buy it. After awhile, Hank decides to tell his daughter the truth and he explains to her that Janet (who became Wasp) did not die rather vanished into a subatomic quantum realm. Hank goes on to warn Scott of the dangers of overriding the suit's safeguards.
Later on however, in the battle between Scott and Darren Cross (Yellowjacket), Scott realizes he must go subatomic to penetrate Darren's suit to prevent danger from befalling on his daughter. He defeats Yellowjacket, but it results in him entering the quantum realm which Hank warned against. Scott find a way to get out of it, but he does not remember how when he explains this to Hank later on. Hank is baffled and now questions whether his wife is still alive.
Now, keep in mind that time is interpreted differently once you're in the quantum realm, so for all we know Janet has not probably aged a day. In addition, she might be in a hyper sleep since Scott was slipping into it until of course he found a way out. Therefore, I believe that Hank will try to find away to locate his wife since her death might not be certain after all. As to how, I believe since she could literally be anywhere he might receive the help of Doctor Strange since the quantum realm was teased by Kevin Feige to be connected to what we will see in Doctor Strange.
2. Hope as Wasp
In the mid-credit scene of Ant-man, Hank unveils a prototype suit he and his wife, Janet, had been working on. Since his wife is gone, he finds it fit that it can be for Hope now, to which she replies "It's about damn time". The prototype is a definite nod to the Wasp suit which we will most probably see in the possible Ant-man sequel. Hank stated that Hope had trained in martial arts and she does know how to "talk" to ants so she should be kicking ass next to Ant-man.
3. More Quantum Realm
I believe out of the whole movie, the Quantum Realm is the one concept that has fascinated me, and the implications of this realm in the context of the MCU only heightens my curiosity. The fact that by going subatomic you're actually entering a whole new dimension boggles my mind and I think it's something that should and will probably be explored in depth in the Ant-man sequel. Most probably, I betting this concept will be explored through the search of Janet van Dyne, if the sequel decides to follow that plot.
This is a given, one of the reasons being the confirmation that Ant-man will become Giant-man in Captain America: Civil War. Another reason is the fact that the Pym particles will probably be examined in depth and be engineered to make someone large. Now, I do not believe Hank Pym and his crew will work towards that out of nowhere since the shrinking particles efficient, but maybe a new villain such as Whirlwind can force Scott Lang to become Giant-man.
5. New and Ruthless Villains
And lastly, of course, we should be in for new and more ruthless villains. Now, this is not to undermine Darren Cross as Yellowjacket because I found him charming, quite childish, and villainous. I empathized with him greatly, especially his father and son relationship with Hank Pym (which we see with Odin and Loki and Tony Stark and Ultron). Yet Darren Cross had only one kill count (that unfortunate guy who couldn't keep his mouth shut), and I wanted to see more of that.
Radioactive Man: This would be a nice visual spectacle to witness in addition to Ant-man and Wasp. Dr. Chen Lu was a nuclear physicist for China before he exposed himself nuclear radioactivity which gave him superhuman abilities. Although he is not known to combating Scott Lang's Ant-man it would still be nice to see, and it actually makes sense. You see, Dr. Chen Lu was researching the radioactivity that gave some people superhuman powers, and since the world in the MCU is discovering that there are super-humans out there, it would make sense that a scientist, especially one in China would research something like this.
Whirlwind: This is the second villain we will most likely see for many reasons. For one thing, he was the nemesis for Hank Pym as the Giant-man and Janet van Dyne's Wasp. Hence, his presence would force Ant-man to become Giant-man in the movie, and the fact that he combated against Janet in the comics would mean that he battles Hope's Wasp in the movie. Lastly, his ability, which is to spin his body at super speeds, would make for a great visual spectacle.
Black Ant (Eric O'Grady): A S.H.E.I.L.D agent who is tasked with keeping an eye on Hank Pym's tech. Eric O'Grady has played both sides in his life (good and bad), but him being Black Ant, or transitioning into one would be cool to see in Ant-Man 2. Now, the only problem with Black Ant being in the Ant-man sequel is the fact that he will probably be similar to Darren Cross's Yellowjacket which doesn't scream 'a unique villain'.