Waaaay back around the time the Leviathan theory started, we had a few posts back and forth on possible ammonia worlds, including Quy/Quiness and Merlin (which I think has been terraformed since the FFE days?)
Merlin has been tested. Ammonia world has been tested (maybe not the ideal temp?).
Europa may be an alternative to Merlin, when it comes to ecosystem. It's at least a type of place that would be natural to chose as a dev.