As far as I recall (it's been a while), there aren't really any plans to expand this whole milking thing beyond Veronica and even that is going to take so much time. At least I'd be surprised if we'd see anybody but Diane, Daisy and Veronica - then again, who knows...
Edit:
Ultimately it's one of the big downsides of DC's approach. The game featues so many different topics/fetishes, but almost none will get as much attention, as those players, who are really into it, would like to get. Be it pregnacy content, trans content or milking at the moment. And I'm sure the same will be true for future BDSM content, harem stuff, etc., too.