It is pretty difficult to say there could be a Derasha with much Emes to it regarding the significance of Esrog in this list, as Esrog is just one of the many things that is listed in the Gemara as increasing Zera, and that the Kohen should therefore avoid.

The fact that an Esrog increases Zera could be a subject for Derashah, such as the Tzadikim of Klal Yisroel being compared to an Esrog, and the fact that Klal Yisroel is blessed that they will be numerous like sand, etc.

All the best,

Yaakov Montrose