This is definitely an interesting idea, for which I have quite of an experience. Thus, I am going to add below some questions and issues I believe should be considered with high priority for the bounty system specifically.

• As I see anyone can load up a bounty. What prevents me to be both the owner of the repository and the solver (with a duplicate account) and merge a fake pull request to take away all the bounty rewards?
• It is unlikely that many people will donate their own funds for the solution of an issue, or at least hardly will be the norm. Most probably the project owners are going to be the ones using the bounty system more often. What is the benefit for me as a project owner? Why should I pay the blocktrades fees and eventual merge-rewards fees when I could just have a direct deal with the developers?
• Converting all the funds to SBD comes with the serious issue of losing the original value of the funds against USD. SBD is really quite far from being a low volatility coin, which means 1 BTC converted to SBD hardly will keep the same value against USD. Why should I take this risk when creating a bounty? The same question would apply for the solver of the bounty.
• What happens if the issue is never solved or closed without a solution? When the funds are returned and how?
• What happens if the issue is solved but the rewards never claimed by the solver?
• What happens if one or more donors want to get their funds back before the issue is closed?
• How do you manage eventual disputes?
• All the funds are held by merge-rewards, this is another serious issue. Most of your competitors out there are holding the funds in smart contracts. In this case you will be responsible for the funds, both under a legal and taxation perspective as well as for the biggest issue of keeping the funds secure. You may face hard times with the regulations and you should consider a different approach here very very soon IMHO.

I may have more inputs for you and I'll paste them here as they come to my mind.