A thoughtful place to begin
Look for the thing getting in the way
A helpful buying clue is not only what your child likes building. It is what keeps stopping the build.
If the tower keeps falling and they seem fascinated by that, balance may be the point. If they are frustrated because pieces will not stay together, a connecting system may change the experience. If every build turns into a house, paddock, road or cave, they may be asking for materials that help the structure become a world. If they keep adding ramps, balls or movement, they may be looking for a system that has to work.
And if nothing is getting in the way? That is useful too. Sometimes the current basket of blocks, tiles, boxes or boards is already doing exactly what it needs to do.