Maggie Siff is a good pick, though now she'll play a little old for Honor, esp at the start, given the whole prolong thing.
A decade ago, I might've also have said Kristin Kreuk.
Trick now is a young actress who can project an older soul. Here's other ideas:
-- Chloe Bennet (once AGENTS OF SHIELD wraps)
-- Missy Peregrym (STICK IT, Reaper, Rookie Blue, VAN Helsing)
-- Emily Bett Rickards (once ARROW wraps)
-- Chyler Leigh (if they killed her off SUPERGIRL)
-- Emma Dumont (once she's off The Gifted)
-- Kathryn Winnick (Lagertha!)
-- Sara Serraiocco ( Baldwin from COUNTERPART)
-- going outside the box, Dichen Lachman (DOLLHOUSE, Altered Carbon)
-- going further outside the box, Letitia Wright (aka Princess Shuri)
-- and lastly, give it 5 years and hire Maisie Williams :-)
Any who don't get Honor should audition for Murphy in a new DRESDEN FILES show.
The Ketty Jay stuff sounds neat, but TV's never been kind to steampunk. Witness the fate of THE SECRET ADVENTURES OF JULES VERNE