We run something similar for sports bettors - SBC League.

You write your posts with your picks, you track the results, we verify it and at the end of the month, those members that made a profit during that month get a share of the prize based on the ROI they've made.

Getting the incentives right here is a bit tricky. Since you don't have to pay to play if you start the month with your normal picks losing (the bets with reasonable odds) there's a tendency to make some longshot picks (high odds) and try that way to make the profit.

That is, there will be a lot of participants that will do that when they start losing and the crowd won't show any wisdom :-)

Another problem we had is that some members would stop posting their picks as soon as they've made the minimum number of the picks and they are in the profits. They'd just wait out the rest of the month and take a % of the prize.

And now, the big issue - ego. I imagine that the situation is the same in both the sports betting and trading, there are a lot of experts that are talking about making $$$, but they are very reluctant to have a verifiable track record. I don't see many of the so-called experts taking part in it since it would show their true performance and that's the last thing they'd like the rest of the world to see :-)

As for our specific use case, we found two promising projects:

1. PredictEV (my profile)
2. BlizPredict(my profile)

The second one I found about yesterday. You should check these two to see what can be lifted from them and used for your dapp.

EDIT: @natedogg, the PredictEv CEO just posted about the new updates for the project in our discord server, you should read about it on BitcoinTalk to see what it is about and what can be used for your dapp idea. Just replace the sports picks with trading and you're good to go :-)