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.
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