There are many different editions of siddurim with many commonalities but also many differences, see here for discussion on MY. Also different communities use different siddurim to match their traditions and philosophical positions, What Siddur Do Open Orthodox People Use?. But this question about the differences in the prayers in Ashkenasi and Sefardi siddurim inspired this question.
Were a person to find themselves in the position of compiling/editing a new siddur, what are the halachically allowed changes that they can make?
What I am looking for beyond the questions I have found so far is a forward looking answer. That is, not an answer that tells a nice story after the fact to accomodate what happened but rather a forward looking a summary of what would be halachically permissible.