@bytemaster has stated a possible solution, in which top 15 witnesses do MULTI-SIG to relevant bitcoin transactions, however "In theory the witnesses could collude to steal all deposited BTC funds."
in my words the "risk" refer to this, I don't know whether there are ways to avoid this kind of risk.
The best way to substantially mitigate this risk is probably to have known entities as witnesses. For starters, below is a list of 10 entities that might participate. And there may be other entities that I'm not thinking of, especially in China.
@bitcrab?
The remaining slots could be filled by known entities outside the graphene ecosystem and/or a few of the most prominent, publicly-known witnesses within the ecosystem.
Anyway, I wonder if Dan has secretly been working on a solution behind the scenes. It wouldn't surprise me. Anything announced along these lines would be a pretty big development.
1. Steemit Inc.
2. OpenLedger
3. BlockPay
4. Transwiser
5. Blocktrades
6. Peerplays
7. BitKapital
8. BitGate
9. Chainsquad
10. Busy.org