Sure! Here are the completed sentences using the future with "will" or the correct forms of "be going to":

1. In the future, people **will have** bigger heads.
2. That trash bin is full. I think the trash **is going to fall**.
3. Look at those clouds. It **is going to rain**.
4. This team is playing so bad today. They **are going to lose**!
5. I **will help** you with your luggage. It looks heavy.
6. **Are they going to start** a new business together as they have planned?
7. We **are going to reuse** these plastic bags within this week.
8. Frank **is not going to take** a three-day holiday next month.
9. There isn’t any drinking water left. I **will order** a big bottle now.
10. They brought the paint here because they **are going to repaint** the wall.
11. Today’s generation **is going to live** longer.
12. My classmates and I **are going to join** a community service project this summer.
13. I can’t open this jar. – Give it to me. I **will do** it.
14. Susan feels sick. She knows that she **is not going to study** at school.
15. Mia keeps lots of empty bottles. She **is going to recycle** them.
16. **Are young people going to have** more job opportunities?
17. She passed her exams. She **is going to study** in a university.
18. I think she **will not participate** in the volunteer programme.
19. Addressing global warming today **will save** lives and money for years to come.
20. Air pollution is dangerous for the forests. It **is going to destroy** the trees.