Pregnancy is awesome. I'm not a huge fan of straight up dominance, especially when it involves mother/son incest. Can it be sort of a compromise, where the son has basically been spoiled by the mom and she basically does whatever he wants anyway?
As for my other favorite kinks and stuff, there's the general stuff like blowjobs, titjobs, and anal. Of greater importance for me are breast play (lactation would be AWESOME but not required of course), and eating pussy.
Building of the relationship is great too. Here's sort of what I had in mind for their background and the setup. Let me know what you think:
Dad either left or died when the son was born, meaning the mom raised him on her own her entire life. Because of that, they're extremely close and intimate, and she more or less pampers him. By close and intimate, I mean kind of a momma's boy. They kiss each other on the lips as a greeting and sign of affection, sleep in the same bed, shower together, and are often times (if not ALL the time) nude around each other when they're home. By pamper, I mean she does more or less whatever she can to make her only son happy, which could be a way of transitioning into them having sex.
As for the aunt, I was thinking that she goes through a messy divorce, and so she comes to her sister, the only family she has left, and stays with her as she gets her life back together for an indefinite amount of time. In addition to that, the aunt and the mom were always close, so close in fact that they've actually experimented with lesbian sex when they were still living together. To start all the fun smutty stuff, I can think of a few options:
a) the son catches the mom and the aunt rekindling their incestuous lesbian relationship one night and bugs his mom the next day
b) already being so close and intimate with each other, and with the son going through puberty, it was only a matter of time before the son made an advance on his mom, either in the bed they sleep in together or in the shower, or wherever. when the aunt arrives, the have to hide their sexual relationship from her
c) the aunt is a flirty, playful woman, so when she comes over to stay with them, she teases her nephew and he starts to flirt back after a while. before either of them realize it, things have progressed to a point where sex is the only option. again, they must try and hide their sexual relationship from the mother
d) some combination of the above
So yeah, that was a bit long and winded, but I wanted to get all my ideas out there, and then the two of you can change, delete, or add anything to what I've said.