I am brain-damaged, and a bit slow to actually figure things out, BUT

It seems to me, with the exception with BL-TPS, that the mind behind the Borderlands franchise really likes the idea of Sirens.

Like I said in the title, I do have brain-damage, but it finally dawned to me when Tanis was revealed to be a Siren, that someone in Gearbox really likes sirens.

… just saying…