Malfurion is an excellent healer at all, but it can also threat enemy heroes sometimes. Everything depends on how the game runs and how your party plays. So, I just come up with two builds that will help you out.

The first build is a combination that provides the necessary talents to be a full healer. In fact, this build is very helpful since you will have 4 ways to heal your allies (Regrowth , Healing Ward , Storm Shield and Tranquility )

The second build is helpful when you are the only CC in your party or you need to solo a lane by yourself (or, for some other reason, you need to be more threatening in the game). The changes in this build are focused on Moonfire and Entangling Roots . However, I would not change Tranquility , Life Seed and Storm Shield , since you are a healer yet and those talents help not only you but the whole team.