I was going to propose that inner locations have some way of keeping track of
how long they've lain empty and their province unvisited. After a year or so
monsters could move in. I'm not really sure of how this is handled currently.
The disadvantage to this is that it opens things up to "farming" inner locations
by players who know the criteria and bother to keep track. I guess this book-
keeping could be supplemental to monthly random refilling of X inner locations.