Everyone has interesting ideas.
Here's mine.
Yuri: Hopefully, after seeing Kumi and Kasumi going off to California, she'll get used to all the aliens bumping out of the skies. Probably become some professional alien-catcher or something. Otherwise, she might just stick to doing some household chores. Sounds ordinary.
Kumi: Since she's so enthusiastic about alien-catching (though not as much as Kasumi), I'm sure she'll want to kick a heck of an alien. Can't imagine in the military, so I guess she might become another professional alien-catcher. Otherwise, she might be like Megumi, become a school teacher and become an alien-party advisor.
Kasumi: If she has lost her yellow knife's powers, she'll most likely become a multi-tasker, trying to fulfuil her parents' expectations placed upon her. While she might finally settle on a certain interest of hers, I've a feeling she'll most likely go off to find her brother.
But, if Kasumi hasn't lost her yellow knife's powers, she'll most likely go on to dominate the world and then the entire universe!! *being sarcastic*