I personally headcanon Alicent as having black hair and gray eyes, mainly because it would pit JRRT's beauty standards against GRRM's and I find that funny.
While the Greens are written as being less nuanced and competent the fact most of the Blacks are also horrible people makes me think its more of a "plague on both your houses" on GRRM's part.
I hope they don't make Alicent or Rhaenyra neurotic, especially because their children die. Its plain sexist, not to mention a waste of a character, how women in Westeros always seem to lose their grip on sanity or isolate themselves (Rhaenyra, Helaena, and even Alicent by the very end in F & B, Lysa, Alannys, and to a lesser extent Catelyn as well as Cersei in ASOIAF) when they lose a kid while the men get to rage and act on their feelings (Daemon and Aegon II).
Furthermore, I actually like how Alicent is essentially an older Margaery in Cersei's position as queen dowager and stepmother to someone she despises. Furthermore, it speaks in her favor that she's pretty much the ONLY person to try to make peace throughout the war. Her first terms, which Glydayn understandably labels "generous", guarantees Rhaenyra, Jace, Luke and their descendants financial independence from the Iron Throne (the fact it was considered "an article of faith" among the Greeens that the Velaryon boys were bastards makes it a big deal that they'd be willing to accept them as trueborn to avoid conflict), Aegon and Viserys places at court as well as opportunities for future advancement (albeit with the caveat that they'd also be unofficial hostages), and grants amnesty to ALL of the Blacks while the best Rhaenyra can bother offering in response is "surrender and I'll spare my half-siblings lives". No mention of amnesty for the rest of the Greens, no guarantee of a place at court for the Hightower-Targs, etc.
Later, Alicent offers to call a GC when KL falls and, around First Tumbleton I believe, she even offers to split Westeros in half between Aegon II and Rhaenyra (which would make for a fascinating what-if in my opinion). Say what you will about the merits of each individual proposal but at least Alicent tries to come up with a solution. Rhaenyra's response is always "total surrender or nothing". (The fact she was only willing to send terms to the Lannisters, Hightowers, etc. AFTER all her half-brothers were dead and even then only at Corlys' urging in response to Daemon's inane suggestions, which would necessitate the death of thousands, considering Daeron was in the field with an army and the Ironborn plaguing the coast of the Westerlands, speaks to her desire to pile up the casualties.)
As for Rhaenys, I really hope they give her something to actually do. Glydayn describes her as having lived "fearlessly" and that Meleys was "no stranger to battle" but where are the details? Did she take part in Daemon's Stepstones campaign for example? This is where GRRM's unequal focus in F & B (the first ten years of Jaehaerys I's reign as opposed to the other forty-five to give one example) as well as his refusal to go back and expand The Heirs of the Dragon + The Dying of the Dragons the way he did The Sons of the Dragon really diminishes the material and, conversely, gives the show an opportunity to elevate what exists rather than worry whether or not it can live up to said material as was the case with AGOT.