I'm not sure whether a reputation score is needed and I'm increasing;y unsure whether there is even a problem at all that needs fixing. The allocation of rewards among participants is largely zero sum and the system is inflationary (and will likely be inflationary in practice even for SP holders) which means anyone who gains less-than-average rewards (including dumb bots, at least relative to smarter bots) is actually losing.

I guess the idea of a reputation is to penalize those who participate but make bad predictions more than those who don't participate at all, but I'm not sure that is even desirable from first principles. I suppose you could say that people who don't even have any SP are also spectators but aren't penalized, so that would be an argument for it.