I tried to prioritize actors who looked as close to the physical descriptions we’ve been given so far as possible. If anything has been said about their voice, I definitely gave that priority since I’ll be reading their lines in that person’s voice haha.
(In general, I went with older actors for the six older gods. It seemed reasonable even though the gods can change their look at will. Pictures provided for those who I think are pretty good matches with their Olympian’s description)
Zeus - has blue eyes, black hair speckled with gray, symmetrical lines on his face, and an unforgettable voice - The unforgettable voice bit basically meant one person for me: Liam Neeson. It also doesn’t hurt that he already HAS played Zeus and I have that biasing my opinion hahaha. More specifically, I’m going with Liam around his Qui-Gon age (about 50)
Poseidon - has green eyes, tanned, laugh lines around his mouth, aging-copper hair and a trimmed beard, and a very deep voice - I picture Poseidon to be fairly muscled but not to the extent that Ares is so I choose Gerard Butler. The voice bit seemed perfect for James Earl Jones, but Gerard I think is closer over-all.
Hades - fair skin, dark hair and eyes, and a well-defined jaw. Man, did I want Tom Hiddleston to be Hades. Hades sounds so similar to his portrayal of Loki so far in the MCU. His eyes and hair are a bit too light though. However, one person in the MCU does have dark hair and eyes and a penchant for trouble and tons of charm to boot: Robert Downey Jr.
I haven’t seen him do much of the sentimental scenes Hades has been depicted in lately, but I think RDJ will make a fine Hades.
Hera - soft face, fine features, deep blush, caramel-brown hair, sparkling brown eyes - I don’t have a satisfactory casting for Hera yet, but the closest I’ve come is Lena Headey. This is primarily based on her portrayal of Cersei in Game of Thrones and Margo in 300. The physical description is a bit off but man, she knows how to pull off a proud, condescending queen.
Hestia - wild red hair, small features, fair skin, freckled We haven’t really had much interaction with Hestia but she sure sounds a bit scornful and haughty. I’m going with Sarah Rafferty for her.
If you haven’t seen the TV show Suits, please do so and watch Sarah portray the amazing human being that is Donna Paulsen 
Demeter - strawberry-blonde hair adorned with flowers, tanned, dark blue eyes Again, I don’t have anyone who fits that description perfectly (the dark blue eyes in particular) but Demeter’s gentle and teasing demeanor made me think of Kate Mara for some reason. Her hair isn’t as red as Lafferty’s but she’s younger than the other older Olympians. Oh well haha.
Athena - cool grey eyes, wavy/messy brown hair, plump lips, high cheek bones - Athena is clearly someone we’ll be seeing a lot of so I had to go with a personal favorite for this one: Emilia Clarke.
She fits the description for Athena pretty well In my opinion. I also think Athena’s emotions can be mercurial at times (like that scene when she pretends to cry in the beginning). She clearly has enough control over her emotions to use them to her advantage. Emilia (and her super expressive eyebrows. Look it up haha) might have that versatility in her.
Ares - dark hair, heavily muscled - Jason Momoa. Khal Drogo as the God of War? Yes please 
He would also make Ares more physically imposing than Poseidon.
Aphrodite - long silky black hair, pink lips, Asian features (?) - The Asian bit made me think of Marian Rivera. She’s a Filipino actress/model who gained fame as a shampoo commercial model. Long, silky black hair? Why not a shampoo model lol.
Hermes - young with curly-blonde hair - I didn’t think I knew any actors with curly-blonde hair until I looked it up. I immediately regretted doing so…
Heath Ledger… God, he died far too young. His Ulrich in A Knight’s Tale is the closest character I can think of for Hermes.
Apollo - tanned, messy blonde hair, playful - why not a young Leonardo DiCaprio? Specifically, this one:
I mean, come on. He would also add another Oscar to this star-studded cast =D
Artemis - tanned, wavy blonde hair, jovial - As an Artemancer, I am actually hesitant to try to come up with someone for Artemis. It’s something to do with wanting her to be my MC’s chosen-yet-possibly-unrequited RO. I also can’t think of an actress with wavy blonde hair. If forced to choose, I’ll go with Daisy Ridley. She seems like she can play the joyful Artemis well >_<
Hephaestus - large - I have to go with Dwayne Johnson. He’s a pretty large guy lmao.
Yep, that’s my fancast haha