There are at least 2 other blockchain based prediction markets currently in development Gnosis and Hivemind and more will likely follow. Should they be viewed as on topic for the Augur Stack Exchange site? Since the Augur concept originated from the Hivemind (formerly known as Truthcoin) whitepaper it seems logical to consider it on topic.
Augur is unique, complex and will dwarf the size of all other decentralized prediction markets combined. It set crowd funding records, has a community orders of magnitude larger than Hivemind and Gnosis combined (based on Twitter and Reddit data) and will likely be the first to achieve scale.
People will come to Stack Exchange looking for "Augur" more often than they will come to Stack Exchange looking for "prediction markets". Which other prediction markets (if any) are on on topic should remain an open question to be decided by the community during beta. However the branding of this site should remain "Augur".
Hivemind plans to use an oracle system very similar to Augur. Due to their shared history and economic model, I think Hivemind should be on topic.
Gnosis is much simpler than Augur because it uses a centralized settlement system vastly different than Augurs decentralized oracle model. I am not sure if Gnosis should be on topic or not, but its case is less strong than that of Hivemind.
Decentralized prediction markets require economies of scale in order to be successful. If being more inclusive by allowing the discussion of other markets will help scale the decentralized prediction market Stack Exchange community, then I am all for it.
Just as was the case with Betfair, Intrade, etc (centralized P2P prediction markets) consolidation will likely occur in the long run anyway with one or two leaders capturing most of the decentralized prediction market.
I do think that it is essential for Augur and other decentralized prediction markets to have a separate Stack Exchange distinct from the Stack of any specific blockchain.