follow vs following
The choice between follow and following is subjective. Either could be used and be correct.
Although if following were used, the sentence would need to be rephrased in order to drop to be:
There is no greater thing than following your passions in a way that serves the world and you.
The conjugation of to serve has nothing to with passions.
The following subject is what determines the singularity or plurality of the verb:
in a way that serves
in ways that serve