I will be massively disappointed if it's anything other than the main story of the film. The Ant-man trilogy started with Scott's relationship with Cassie and that's been the most consistent through-line for the character since then, too. With her now also aged up to fit the role (after explicitly talking about her desire to be her dad's partner in the last film) there is really no other acceptable way to end the Ant-man trilogy than bringing the Cassie storyline to full fruition.