I’ve had a bit of a think about this and I’ve realised it’s tricky implement … I feel like the most sensible approach is to allow plugins to take it in turns to replace / filter a transaction. When one filters it then act as before, but when one replaces it then this should again be run against all the other plugins to see whether they would go further a this point. This perhaps is a touch like the ability for appendTransaction to bounce into an infinite loop but it seems to rely more heavily on ordering than previously.
I’m not sure I quite understand why it would return an array here too?