Prediction for the identity of the female darkin, as Varus stated he is looking for his "sister"
Irelia. She's getting a vgu after Swain. Unless they create a completely new champion before that, Irelia fits the bill for a Darkin-wielder - though in her case, she might (or even should) be in control of the darkin weapon similar to what Kayn is attempting to do with Rhaast.
She has always been portrayed as an extremely skilled warrior with a strange, magical weapon controlled through her will. The Darkin are all (to date) promiscient warriors - and always weapons with unusual qualities.
While i do not play Irelia myself, i still like the character - and i hope that, if she is in fact the next Darkin, she is either dominating the weapon or it is willingly obeying her. I can't think of many champions better fit to be a master over a Darkin, and i feel her playerbase would feel betrayed if they lost the identity of the champion they love.
EDIT: Separate from my theory on Irelia, i am sort of hoping they are filling out a full team here. We have Aatrox for the top lane, Kayn in the jungle and Varus as a marksman. Morgana would make sense as a support Darkin, though her current iteration does not wield any physical weapons - neither does Tahm Kench, the other champion frequently suspected of being potential Darkin.
Now that we know the Darkin are bound to weapons, Morgana seems less of a suitable candidate.
BUT! And here's where we put on our tin foil hats.
Varus was sealed by a queen in golden armor. League of Legends feature two champions in golden armor, one of them in full plate, waiting for a VGU and not tied to another lore act:
.
This brings Morgana back into the picture. This, to me, seems like a PERFECT opportunity for their eventual reworks - as Riot has confirmed these will be recieving dual reworks simultaneously released.
This does not give any indication of who the fifth and final Darkin might be. I seem to remember a Rioter confirming the Darkin consist of four males and one female, and as such - if they are to fill out a complete roster - i am hoping for a new mid laner. (Or, if this fourth Darkin is a mid laner - a new support.) so that all five Darkin can be played on the same team effectively.
Bonus point for the skin potential.

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