No one except God knows the future, of course, but almost certainly it will be different from today. That means those who follow us will have their own challenges and temptations, including some we can't even imagine.
No, it isn't silly to pray for those who will follow us -- even those who haven't been born yet. Repeatedly over the years, I've met men and women who had a godly grandmother or saintly great-grandfather whom they never met, but who prayed for them and for others who weren't yet born. These men and women were convinced that their own commitment to Christ was a result of those prayers, and I can't help but agree. This may have been what the Psalmist meant when he prayed, "I will perpetuate your memory through all generations; therefore the nations will praise you for ever and ever" (Psalm 45:17).
At the same time, pray especially for those who are already in your family -- for those who need Christ, or are facing special trials or temptations, or need God's guidance. And pray for others, as well, even people you've never met, such as your church's missionaries, or believers in other parts of the world facing persecution, or evangelists who preach in hard places.
Prayer is one of the greatest privileges God has given believers, and it's possible only because of what Jesus Christ has done for us. Through our prayers God "is able to do immeasurably more than all we ask or imagine" (Ephesians 3:20).