As crazy as this sounds... I think more than an "actress" as a character Sue needs to have a big, warm, confident and intelligent personality.
A BIG personality. I think all of our team need to "pop" onscreen but of them all Sue is the one that should get attention simply because right out of the gate she's not as "flashy" as the others outside of how powerful and versatile her powerset is. But... She's not "the world's greatest scientist", she doesn't have Ben's obvious yin/yang of blusterous heroic catch phrases and gravitas filled angst. And she's not factory equipped with Johnny's made for the MCU flippancy and one liners.
I would say before thinking about "prestige" types look for the actresses in their early thirties to early forties that really inhabit the screen, who stand out. Look at Alba... There's just no there there in the Story films, and no, not just because of the writing. It's not just that Alba phones everything in either. Outside of her seductive sexual appeal... There's nothing else. She's invisible as a character even before she gets her powers.
MCU Sue has to be someone that we buy being the only person that can lure Reed Richards away from breaking new barriers every second of the day because she grounds him, calls him on his ****, and will fight tooth and nail to protect her man and the rest of her family.
In fact... Here's something I think will be unpopular but... Seeing as how much humor is given primacy in the MCU, and seeing how Reed probably shouldn't be a clone of RDJ's Stark, I say give that kind of cutting, biting wit to Sue as a character trait. Ben and Johnny can still be the pure comedic duo but I think Sue could be way more in the Frasier Crane type of humor set where Ben and Johnny can plow the fertile feilds of low brow humor. This puts her more in play too in scenes of dramatic confrontation if her voice is like Stark's, esoteric in referrences, cut to bone withering and both playful and sarcastic at the same time.
(As always I note... None of these things requires her to be a scientist, but I digress.)