After several months pondering how to make the best possible plugin architecture, I have come up with an answer. The code is still in an infancy stage and will require work before it can be applied in "real" projects (PM me if you do want to use this and I'll tag along).
What I've made allows you to load an Au3 script file at runtime and call methods in it. This script doesn't have to implement or change anything. Combine this with other functionality such as what Eval offers and (shameless