• Beam spot constraints, prior information, ghost tracks, vertex suppression field, and other "extra information" needs to be respected in the interfaces, without bloating the API.

• Zvres and a special-purpose MultiVertexSeeder? for the application of b-tagging are desirable future developments.

• Gaussian sum filters will be introduced, allowing for non-Gaussian track models. (Think e.g. modelling bremsstrahlung electrons here)