Actually, you can do rule modules... If it's freeform. (ie. You pretty much just add fluff restrictions.)
Or, DnD4E and PF1E can probably interchange with minimal work if you don't care about balance and stuff. (re: The overusage of additional rule modules in The Great 4E Splatbook Surge, and then The Great 4E Splatbook Flops.)
Otherwise usually the closest people can get to interchangeable rule modules between completely different systems are Achtung! Cthulhu... Where they literally wrote 2 rulesets (CoC/SW) inside a single book.