"Drop-in" is what enterprise software calls "bug-for-bug compatible" - e.g., replacing RedHat with CentOS (RIP) should work exactly the same, even if the CentOS team found bugs - they report them upstream and do NOT fix them themselves, because code may be relying on the bugs.

This is especially true with complicated vanilla Minecraft setups and red stone machines (Java Minecraft red stone has "bugs" that "shouldn't be there" but cannot be removed now since so much depends on it).

Guess this gets to my other point.

By the time you are 'compatible' then you have implemented everything needed to also be a 'drop-in' but data file formats might need a conversion.

So convert from Minecraft data files to Pumpkin data files. Then it could drop in.