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I'm looking for sci-fi fantasy book recommendations for someone who loved the Harry Potter series, Wheel of Time, Game of Thrones, and everything by Brandon Sanderson!
I love reading about well created worlds or any sort of well thought out magic system

Maybe Artemis Fowl? If you liked Harry Potter, that's kind of in a similar vein. I've only finished the first book in the series, but what I've read I adored. My husband read the entire series. He claims it can be a little "preachy" about its environmentalism themes, but is overall a decent series with interesting characters.

I think you should consider reading the Shatter Me series by Tahereh Mafi.

The Shannara Chronicles by Terry Brooks, The Sword of Truth series by Terry Goodkind, Incarnations of Immortality series by Piers Anthony, The Dark Elf Trilogy (this became a much larger series than just the original trilogy) by R.A. Salvatore, Ender's Game Trilogy by Orson Scott Card. Honestly, I could list tons. If you try any of these, let me know how you like them.

"The Knife of Never Letting Go" or "The Colour of Magic." are both amazing for fantasy world building.

The Jane Hawk series by Dean Koontz is great!

This is a somewhat obscure fantasy series that I found at a used bookstore: The Stormlord Trilogy by Glenda Larke (first book is The Last Stormlord). It uses a magic system that primarily revolves around water and storms because of the desert setting and water shortages in the land. The main characters are a village outcast who has to leave his home and a slave who escaped a life as a courtesan. As a fan of Robert Jordan and Brandon Sanderson, I highly recommend these books!!

I read and loved the Harry Potter series and Game Of Thrones series. I plan to read the wheel of time. After I finished reading GOT, I was searching for an equally good fantasy book. That's when I read the Assasin's apprentice, book one of the Farseer Trilogy by Robin Hobb. I like it better than game of thrones now. Give it a try, you won't regret it.

I would highly recommend the Wheel of Time by Robert Jordan. Amazing series, so in depth, well written and the world building is fantastic. I've also enjoyed the Diablo book series (if you have ever played the game) the books are quite good especially the ones by Richard A Knakk. Also, Piers Anthony, Mercedes Lackey, James Mallory, Troy Game series by Sara Douglass,

I'd recommend the 100 Cupboards trilogy and The Dark is Rising. Both are very similar to Harry Potter or Brandon Sanderson (I am also a Sanderson fan!), but they are lesser known. I think you'd enjoy them!

Try Masters and Bastards by Christopher J. Penington, I'm sure you'll love it.

What a great list for myself to work on as well!
I will add Mistborn add my contribution. I'm working on book two....

Anything and everything by Raymond E. Feist. All his trilogies are set in the same two worlds years apart from each other. The characters in one trilogy are the grandchildren of the characters in the first trilogy. The characters in one trilogy end up becoming influential in the kingdom and help fight the evils that plagued / still plague the characters from the first and second trilogies. Absolute genius writer!

Since you mentioned Game of Thrones and well-thought-out magic systems, you should try The Name of the Wind by Patrick Rothfuss if you haven't already. There is a sequel to it also titled The Wise Man's Fear. I enjoyed both of those books by Patrick Rothfuss, they were so fascinating.

Another one with a good magic system is A Wizard of Earthsea by Ursula K. Le Guin.

You also mentioned the Harry Potter series - in that case, you may like The Lightning Thief series by Rick Riordan.

Glen Cook's The Black Company series!!