I'd like to hear from anyone who has built a dialect continuum, in which innovations spreading from different centres affect overlapping subsets of the range (see wave theory). How do you model it? How do you manage the results?
I suppose I'd start with a list of towns and a table determining how likely a town is to adopt an innovation from each of its neighbors. The table can change over time as towns lose or gain influence, or new roads are made.
Then what? Maintain a separate grammar for each town? Impractical.
With a list of sound shifts, you can easily generate a given town's version of a given lexeme as needed; but managing changes in syntax or morphology is, I'd think, not so easy.