As of the end Skin Game (and presumably into the beginning of Peace Talks - I haven't read the released chapter), six known entities have the knowledge to perform the Darkhallow: Harry, Mavra, Cowl, Evil Bob, Ivy, and Lasciel. I'm assuming Grevane is completely dead and not returning. Also assuming that Kemmler is staying dead. Think we'll see someone successfully pull it off?
I doubt it will be Harry. He's already gone down the Winter Knight path for power, and isn't overly thrilled with the consequences.
Mavra might not be interested in it. Her desire for the Word of Kemmler may have been to remove knowledge of her weaknesses from the world instead of a power grab. Her motivations are unknown to me at present.
I still think Cowl wants the power, but the circumstances haven't aligned for him to try again yet. He's a contender.
Evil Bob wouldn't (I think) be able to do it on his own behalf, but after Capiorcorpus went southbound, I imagine he's looking for new allies.
Ivy knows everything that's ever been written down, but I seriously doubt she'd ever use it.
Then there's Lasciel. IIRC, she knows everything that the shadow Lash knew (possibly knows?), so when Harry scanned each page of the Word for her to memorize before Grevane took the Word, a copy of that info went to the Fallen angel proper. As of Skin Game, Lasciel is evidently collaborating with Nicodemus, putting the knowledge of the Darkhallow in the hands of the Denarians. Would any of them use it?
Oh, I guess Anduriel might have already known it, too. Master of Shadows, and all that...not definitely, but possible.
Unknown if others like the big players (Fae queens, Drakul, Dragons, non-Fallen Archangels, Outsiders) have any knowledge of it, but I'm guessing not.