Seriousness rhymes with:
rebelliousness, callousness, carelessness, cautiousness, consciousness, contagiousness, contentiousness, dangerousness, disingenuousness, fractiousness, graciousness, hellaciousness, helplessness, homelessness, hopelessness, joblessness, lawlessness, nervousness, outrageousness, pompousness, powerlessness, rebelliousness, recklessness, restlessness, righteousness, ruthlessness, surfaceness, weightlessness
Seriousness sounds like:
saracens, scorching, scourging, screeching, scroggins, searching, sharking, shearson's, shirking, shrieking, shrugging, sourcing, surgeon's, surgeons, surgeons', surging