Here are some possible completions for each sentence based on the context of the wishes:
If I were Mitch, I would save money to travel abroad.
If Mitch hadn't gone to Rio, he might have had more time to study for college.
If more people volunteer, the community would become a better place to live.
I wish I were more confident in social situations to make friends.
I wish I had experienced more cultures while traveling abroad.
Feel free to modify any of these suggestions to better suit the perspective you're looking for!