Impressive. If it can handle two-headed walktapi and other wacky body configurations, you're doing a good job!
(I know - I did an RQ monster-generator prog years ago, which included that sort of thing).
The rules I do use are more like old-style BRP - i.e. they don't include the opposed-roll "Attack/Parry Matrix", but each side rolls independently. Also a different HP formula, and Major Wound/Death thresholds (i.e. characters can go into negative HP). Can Fantasy Grounds accommodate that?
My concern about VTTs is that they can be overly prescriptive. Mainly what I'd want is just the display & communication, perhaps with dice-rolling added, and possibly monster HPs/stats. (Characters etc can be separate text files or whatever - I'm not convinced incorporating them is an advantage).