This is going to be the next big addition to the city generator. To be exact, I'd like to make it possible to generate cities on any land configuration including islands and river confluences.
Viewing post in New Map Types?
I don't know how exactly parametrization is going to look. Most likely a couple of new options will be added (e.g. "island", "confluence", maybe something else) in addition to existing "river" and "coast". For anything more exotic (including multiple rivers) there will be an option to upload a "water map" image. I just hope the UI won't be too complex, I hate spending time on buttons and checkboxes...