Some thoughts on modularity:
Your system and 5e use "hit dice" as a psuedo difficulty marker. A system like SW lacks that, or has an entirely different stat (or calculation) to do that.
Bring systems in with a standardized stat block, and allow the "difficulty market" to be set based on system, or turned off entirely.
If a system lacks a modular monster build system, you can build in some light re-skinning. For example, there tends to be ton's of generic NPCs such as shop keepers, guards, etc. You can flag those as "generic" and apply new names as necessary.
For monsters, the same could be true, but there might be some back-end list work. Build a list of creaturs/monsters that could be represented as the same stat block in man games. (for example, Big Cats: Lions, Tigers, Lepords, cheetas, etc.) Stretching farther, you might even be able to extend that same stat block to things like bears, and other large predators.) If the creature doesn't exist, you can use a "generic" flag, and the add whatever name you need there.