I tend to understand the adjective 'ważnego' is in genitive case because it's more close to the English alternative construction of this sentence " it is something OF importance"
At least that's the way I'm going to remember it. Hope this helps.
It can't be -y, because coś is neuter.
The question remains why -ego not -e then. And the answer is: because it is so. Adjectives with coś are usually always in genitive, regardless of case coś is in.
Note that ktoś is masculine and with it, the case of adjectives always matches.