Handsome Jack only eludes to a "6 siren limit", he never comes out and completely says it. Nor is it mentioned really anywhere else. Another thing is, Ava was on the Siren's monestary planet, she was being trained in the ways of the Siren.

Second thing, one would assume that during the Calypso's "birth" the game says they were locked away because of their powers. This time would also presumably break the rule y'all all seem fond of.

Lilith got the worst of the playable powers imo, her second game form was much better. Also iridium was unknown since the Destroyer was sealed and iridium with it. So then being tied to iridium makes no sense. I wouldn't mind seeing a leech ability, but ngl I'd absolutely love phaselock again too