Should not be a problem overall. How is it any different from layering stacks of other terrain procedurals within Terragen 2. A plug-in would work in a similar fashion, in this case there is just many more procedurals at work here. It would be semi-hard coded into Terragen working code taking advantage of Terragen's code. Were just adding more to the table in terms of terrain variety. I have no doubt that we will have better procedural techniques in the future but these third party solutions have a lot to offer in terms of realism. For example i can mix several erosion styles by altitude, slope, roughness within GeoControl like you see in nature or i can go into World Machine and create fairly convincing faulting were the terrain looks as if it were shifted by many earth quakes.
I am not saying building a planet size heightfield within these programs then importing it into Terragen 2, that is insane. What i am saying is building these procedurally within Terragen 2 while the plug-in shares itself with Terragen's code.
It is true that more complex calculations will take longer to process but computers are getting faster and newer cpu chips, video cards and ram is becoming cheaper every few months. A heavy sediment erosion would probably be the most heavy to calculate out.
The Alpine Fractal is limited in terms of basic fluvial/water erosion were as in the third party programs there is thermal, glacial, high sediment, inverse, terraced, wide flows, think flows, deep-cut, hard and soft erosions.
Imagine having a full blown river tool inside Terragen 2. I mean MojoWorld has a River Plug-in for a planet but GeoControls blows it away entirely.