There are various synonyms for the phrase "for one thing" that can be used to show a transition between two points. Firstly, the phrase "firstly" can be used to indicate the first point or idea in a sequence of points. Secondly, "in addition" can be used to add another point or idea to an already established sequence. Another synonym is "moreover," which is a more sophisticated variant of "in addition." Fourthly, "furthermore" could be used when one wants to build on the already stated point. Finally, "not to mention" is often used to introduce something that one has left out or forgotten to mention earlier.