You gave the example "so do I". It's the same as the other examples:
"Ed wants to go to Spain for our holiday, and so do I ___".
This is just like the examples I gave you with "so will Max" and "nor can I", where "so" is seen as a connective adverb. In this example, the ellipted predicate (marked by gap ___ ) is "want to go to Spain for our holiday".