Some sort of quick change plug to make cleaning and such easy.
I am big on stainless steel in genera and for a gun that is likely to see corrosive it is a MUST IMO.
I bought a used TC stainless for $150 off craiglist a few years ago. Used muzzleloaders are a great deal. THe only caveat is there aren't a lot of breach plug models on the used market.
Conversation starter at the range... Yes, but in my experience the conversation is something like: **Cough, COugh** "Put that **** smoke pole away" repeated as the cloud of smoke floats past every station on the line.