With the version number I would like to indicate that the approach taken in the package is mature and battle-proven, and further significant developments would likely be based on a new approach.
I intend to keep maintaining mkin 1.0 and will happily fix bugs if you report them. Therefore, mkin 1.0 may be a suitable candidate for building a graphical user interface, as it seems that only few scientists working in the area of regulatory environmental fate modelling are inclined to work with R scripts.
Personally I am in the middle of a project exploring the application of nonlinear mixed-effects models to degradation kinetics. This is currently also based on mkin, and in fact, an interface making it possible to set up such models using mkin is already there. However, there is still a lot of work to be done before this approach can be recommended for regulatory practice. If you are interested, please don't hesitate to contact me!