I'm looking a writer/thinker who has ideas on how to preserve classically liberal & democratic values in the 21st century

who should i check out?

TLDR; WEB Du Bois, MLK Jr, Kwame Ture, Malcolm X, and so on through today's leftist leaders like Kshama Sawant and Chris Smalls. (For a US centric answer)
To explain why:
The liberals have shifted to conservatism in an alliance with fascism to maintain the status quo (which I describe as Oligarchy).
So if you're looking for where the movements for progressive change in society are going in the 21st century I would look to leftists like socialists and communists. While liberals focus on maintaining order, leftists would focus on using our collective resources to meet peoples needs, responding adequately to climate change, creating international solidarity and mutualism and building towards peace, and expanding human rights to all people. Today's leftist leaders carry on the torch from the American Black radical tradition, it's allies, Indigenous resistance movements, Labor struggles and international leftist leaders of history (all with a rich history of good books).