The Honor Harrington series. Would have to be 3d animated IMHO- One major component of the books is humans have received lifespan extension treatments which greatly slows down human aging. So you would need a ton of 18 to 25 year-old actors who can pull off playing 50 and 100-year-old characters.

Philip Jose Farmer's Riverworld series could be a fun series/world to adapt to a series, but also infuriating if they cut it short after two seasons.

The Scholomance series by Naomi Novak could be great.

Would love to see an Otherland adaptation into a series. There was a film being planned but haven’t heard anything in a while. Tad Williams is a fantastic author.

Wraeththu by Storm Constantine.

The Hyperion books by Dan Simmons.

Melanie Rawn: The Dragon Prince

I didn't think special effects could do it justice in the past, but I think they've definitely advanced far enough now.

Bobiverse series would be relatively easy to adapt, Children of Time by Adrian Tchaikovsky not do much, looking forward to Bradley Cooper's Hyperion adaptation

Jeff Long's The Descent:
In a cave in the Himalayas, a guide discovers a self-mutilated body with the warning "Satan exists".
In the Kalahari Desert, a nun unearths evidence of a proto-human species and a deity called Older-than-Old.
In Bosnia, something has been feeding upon the dead in a mass grave.
So begins mankind’s most shocking realization: that the underworld is a vast geological labyrinth populated by another race of beings. Some call them "devils" or "demons." But they are real. They are down there. And they are waiting for us to find them…
And it's sequel, Deeper:
A decade has passed since doomed explorers unveiled a nightmare of tunnels and rivers honeycombing the earth's depths. After millennia of suffering terror and predation, humanity's armies descended to destroy the ancient hordes. Deep beneath the Pacific Ocean, a doomed science expedition killed the subterraneans' fabled leader, and suddenly it seemed that evil was dead and all was right with the world again.
Now "Deeper" arrives to explode that complacency and plunge us back into the sunless abyss. Hell boils up through America's subways and basements to take its revenge and steal our children. Against the backdrop of a looming war with China, a crusade of volunteers races to find the vestiges of a lost race. But a lone explorer, the linguist Ali von Schade, learns that a far greater menace lies in the unexplored heart of the planet. The real Satan can't be killed, and he has been waiting since the beginning of time to gain his freedom. Man and his pitiless enemies are mere pawns in the greatest escape ever devised.

For me its simple: Metro 2033 as a movie. Do I want that real bad.

I know it had a brief go in the 90s, but I’d love to see a proper adaptation of Animorphs. Those books were wild, especially for something I remember starting in 3rd or 4th grade. I’d prefer animated personally, but I would tolerate another live action series. Tech has definitely improved enough to do the alien races justice in either format. Another pick would be Vampire Hunter D. It got a couple anime movies but I think a series could really do it some justice. It has such a fascinating world.

Leviathan by Scott Westerfeld. You'd need a massive budget to make the darwinists look convincing though so it'll probably never happen.

Tad Williams - The Dragonbone Chair

Trudi Canavan - The Black Magician Trilogy

Something by Alastair Reynolds. I think Eversion would work as a mini series. Otherwise Revelation Space would be sick, but perhaps as an animated show instead of live-action.

A Darker Shade of Magic was slated for a movie, then a TV show, but I think it's basically been dropped. I really enjoyed that series and think it would be a pretty easy adaptation.