I would propose the only change that be made to the Trireme is to reduce the population cost, leaving everything else the same. Reducing the population cost to 12 would make it slightly better in defense, while it's lack of speed would deem it still less useful as support, when compared to the Bireme. Light Ships would still be superior attackers. This would make the Trireme a much more viable option, even if only as a colony escort, simply because one could make more of them.
This wouldn't change the Trireme's usage all that much but it would be much more effective when it is used. It would still have the added benefit of building very quickly for the total attack/defense it provides, and it's speed and resource cost would still limit it's use. However, it would at least have a niche where it excels. This change would make the Trireme relevant, but is would not become an end-all, over powered, build-only-this unit.
Reduce the population cost to 12.