It seems to me that a lot of the answer revolves around what do you want to do with it and how good are your abilities and desires in regards to creating and loading challenging cartridges.
If you just want a plinker out to 100 yards then most any centerfire caliber over .34 caliber is fine. If you want to use it for hunting or long range target/silhouette shooting then a rifle cartridge, preferably rimmed is the answer and in the case of BP specific competitions, obviously one designed for black powder. You might want to browse through "Cartridges of the World" to get an idea of the choices.
If you are not a die hard reloader of the obscure then stick with calibers than are currently commercially available like 45-70 and 38-55 but if you are really dedicated then I would suggest something based on a 45-70 shell like 38-56 or 40-70 bottleneck.
cheers mooncoon