I've had to do some casting for videos and short films I've made. The smartest thing that our honorable EPs (Prof. Olsen, Trish, and Dave) could do is cut the casting process into two steps. When most people think of casting for a role, they immediately start thinking of the actor who most looks like and has previously played a similar role to the character being cast. However, the most important criteria for filling a role is that the actor is able to convincingly convey the personality of the character he/she is cast as.
This first process of casting should not just be about what our main roles look like, we should be taking the time to craft believable and distinct character profiles that include motivations, emotional strengths/weaknesses, worldviews, habits, etc. Now obviously our honorable EPs don't have time to contemplate detailed character profiles for every character in season one in a single episode, but this should be the general focus.
This is not to say that physical characteristics are not important. They are important and should be part of the consideration. But physicality should not be the ONLY focus of casting. There needs to be a balance between the right personality fit and an accurate physical portrayal. If taken to the first extreme, you end up with Rupert Grint as Ron Weasley. He's a perfect Ron, but only if you close your eyes. In the books, Ron is weedy and very tall- two things that cannot be said about Rupert Grint. Taken to the second extreme, you end up with George Clooney as Batman. He's unfortunately the closest someone's come to looking like Bruce Wayne from a comic book, but his acting in the Batsuit left fans feeling (wait for it...) a bit cold. Schwarzenegger (as Mr. Freeze from Batman and Robin for those of you who didn't get my [humbly] hilarious reference) is a rare example of an actor who neither acts like nor looks like their literary counterpart. Let's not do that either.
Also, for the Valar, I think it will be important that we pay attention to the actors' voices. Since the Valar will be changing appearance at least once, we should make sure the voices fit the characters almost as if we were casting for animation. Now, practically, we could always change the pre-children designs of the characters after casting, but I think it's still something to consider.
Finally, all of this just to say- feel free to take my opinions with a grain of salt. The only sure way to properly cast a role is to audition actors and screen test multiple times (and sometimes even that fails...) So we really only have an actor's body of work to base capability on, and I implied not to do that in the first paragraph. Also, many groundbreaking roles and famous characters have been played by people who had never professionally acted before or were never famous before that role. For all we know, the perfect actress to play Luthien is your cousin's best friend's former high school prom date's daughter. Who would know? Plus, this is dream-casting. If I had it my way, Jason Issacs, JK Simmons, and Jennifer Lawrence would be playing every role.
So yeah. At the very least, maybe the things above are something to think about while scrounging IMDB for ideas.