Don't know if it is a new Idea or if it is a good idea...but let me put it in and look what happens.
As i'm a really idiot using WRP and Oxygen (but yes, i know breathing ;-) ) i had the Idea: "Hey, wouldn't it be great to have an Island with absolutely no Roads and Buildings on it? That way i could create easily an Island that comes close to my desires!"
You see what i mean? So i speak of an "empty Island" which only have trees bushes forrests, but no...uhm...lets say "human structures". This way, Mission makers could create nearly exactly the surrounding a mission needs. I know there are a few dozens excellent island around, but hey didn't you really never thought "hey cool island, but i would prefer a few houses more over there (already easy to make i know) but a few less over there".
I hope you can see what i mean and i think i know the upcoming problems: however the "customizable island" would look like, it would never be useful for absolutely everyone. But i think a good island maker (and i am definately NOT a good one) could find a good compromise.
And also another idea but i don't know if it is makeable: what about buildings which allows mission makers to easily change the thextures? Let me shortly explain: Let's say a addon maker creates a simple house to place on maps. The addon comes with a standard texture for simple adding into the mission editor. Into the addon download comes also a "texture template" which can be edited (painted) with any painting program and then added to the house (this settexture?). This way you could create towns by using same house with different textures/looks.
Well, these are my ideas. I hope i described them well enough for you to understand what i mean. If one of these (or both) already exists please tell me.