Texhnolyze, Shinsekai yori Recommendations
Both are very dark and original futuristic settings that deal with philosophical themes that unravel through amazingly written twists and mysteries until the truth about their worlds is uncovered.
Texhnolyze is bleaker right from the start and deals with human nature and the meaning of life, while Shinsekai Yori starts seemingly happy-go-lucky and slowly gets revealed as a dark story and deals with morality issues.
Both are grim, nihilistic masterpieces revolving around mankind's "evolution" leading to its implosion. Shinsekai yori does this primarily through thorough examination of power structures and their tendency to divide populaces for control, while Texhnolyze examines how our ambition can run rampant to the point where we do a 180 degree turn and regress back to primal, sadistic animals. Both are essential viewing for those interested in socio-political fiction (though Shinsekai yori is definitely an easier watch, so try it first if you've yet to see either).
Dark post apocalyptic futures and social adjustment are noticeable themes throughout both series. Though texhnolyze maintains a dark tone throughout the show with morbid fight scenes, desolate landscapes and seemingly just a story of one man's survival, shinsekai yori seems relatively happier to begin with, as the main character successfully graduates to a new school with her friends, but discovers all is not right with the world. Social segregation in shinsekai yori comes by way of introduction of a subservient species which is explored further in the show, by contrast, Texhnolyze establishes social non integration through gangs early on in the series.
Both deal with explicit themes and follow through with them. However, Shinsekai Yori is a bit more lighthearted than Texhnolyze, while Texhnolyze is dark through and through.