This is a tricky issue. I have some notes on how to deal with it that I’ll try to finish and post here. Some quick answers though:
- You could do away with limbo exits as well as requiring exit bonds. The tradeoff is that past coin owners can grief current coin owners (but at a bounded griefing factor).
- In the specific case where you are just sending one coin from a sender to the receiver, it is fine to require them to collaborate in case the chain is unavailable. Difficulties arise when considering multi-input transactions (e.g. a change provider).
- The plasma cash variant described at Plasma Cash with smaller exit procedure, and a general approach to safety proofs might be able to support multi-input transactions with bonds as well as multi-input transactions, but I am not sure if the original plasma specification could do so.