An obvious way to save space that is not mentioned is of course not to have alphabet sized pointer arrays in each node and if the alphabet is big, for example U

An obvious way to save space that is not mentioned is of course not to have alphabet sized pointer arrays in each node and if the alphabet is big, for example Unicode characters, it is not really an option to begin with. In that case you would use some kind of dictionary to store the pointers to child nodes. You can also dynamically switch between different node implementations depending on the number of child nodes.