The Psalmist was praising the Lord for what He was doing and going to do in the future. I believe a people yet to be created is speaking of the gathering together of all people of every tribe, tongue, and nation into one people called the church, the bride of Christ. Anything that God says is going to happen absolutely will happen as He can be trusted to fulfill His word and promises. What is it you are worrying about today that God already has an answer to? It may not be exactly what you want it to be but the Father (God) knows best how to do all things so that they work out for our good and His glory. Let us praise Him to for all He has done and will continue to do for us and His children (believers).
Psalm 102:18 This will be written for the generation to come, that a people yet to be created may praise the LORD.