Rather than a straight reduction in the criminal countdown, I'd suggest a cumulative GCC: 2 minutes for initial aggression, add 5 minutes for ship destruction, then add another 8 minutes for podkilling. This would facilitate solo roaming (no more getting stuck in one system because you had to disengage when your target's backup arrived and now don't have enough buffer left to withstand a volley from the gateguns) and provide a modest disincentive against podding; the risk of losing implants is one factor that's often cited as keeping people out of pvp. At present, there's no reason to avoid podkilling once you've initiated aggression if you're not bothered about your sec status. With the proposed change, the attacker would have strong incentives to ransom ships and pods (and to uphold their end of the ransom deal), or to just let pods go unharmed.
edit: to clarify, the maximum possible GCC would remain capped at 15 minutes; the point of the idea is to
i) faciltate solo/small-gang/small-ship roaming by *reducing* the GCC when an engagement has resulted in no (pod) kills
ii) provide aggressors with more of an incentive to refrain from podding their victims, making low sec a bit more forgiving for newer players.