In all fairness, they haven't permanently split up so much as taken a long hiatus, which ends (at the time of writing) this summer with their reunion tour. Fans can only hope this leads to new material, but there hasn't been mention of this from the band.
There's also a LOT of material that never got officially released by the band, mostly appearing on file-sharing networks and bootleg albums. As System of a Down took a lot of time between albums (three between the self-titled and Toxicity, three between Steal This Album! and Mezmerize/Hypnotize), it makes one wonder if they had other albums planned during those times.