I would personally allow free account creation but give people an ingame salary of currency for every day in which they engage into trades.

Developing a good prediction market that's people actually want to use is a bigger problem. PredictionBook sort of works but it could work better than it works at the moment.

PredictionBook already exists and is opensource. If you want you could probably write a plugin that adds prediction market functionality on top of what already exists in predictionbook.