Here's how to make people who invested earlier gain more:
Imaging a user bid 1 BTC on 50/50. This means, out of the pool of JD, 1 BTC is wagered against him (well, actually 0.99 BTC but for simplicity I put 1).
The question is: whose BTC is being wagered against the user? Right now, it is taken from all the investements in equal proportion. What can be done instead, is that it is taken from all investments *weighted* by the time they invested they made the investment. For example, 1/5 BTC from the first month's investments, 1/4 form the second etc. In fact, better use slower decreasing weights to make the investement attractive for new users as well. For example, [1/k(k+1) - 1/N(N+1)] weight for the BTC number k, with N being total number BTC invested.
Obviously, the wins/losses then are devided in this proportions.
one issue with this is that a common pattern could be:
divest all
gamble
invest all
I don't see how that could be a problem. If you divest and then reinvest then of course your investement moves all the way to the end of the line, and will be earning less. Wheter you gamble in between doesn't matter.