I've seen a lot of procedural terrain generators, but what I need is some level of control over it. Is there such a thing as a procedural generator where I can use an existing heightmap as a base and draw out where I want the algorithms to run, as opposed to them just kinda going all over the place?

I've seen a lot of procedural terrain generators, but what I need is some level of control over it. Is there such a thing as a procedural generator where I can use an existing heightmap as a base and draw out where I want the algorithms to run, as opposed to them just kinda going all over the place?

I use simple procedural texture generation algorithm (I've written a little tool for it ).
Heightmaps are just images, using a procedural texture tool would be a easy way to manipulate a heighmap in a controlled manner. The problem is, that most image tools only support 8-bit values, whereas a heighmap often used atleast 16-bit height values.

When you are happy with 8-bit then check out mapzone, load your existing heightmap as grey-scale image and play around with different nodes.