At this time, I believe that editing (changing) what's already there is the only option. What you're talking about is adding additional, new stuff to the game (and it's code).
However, there's already been one update with additions to the functionality of add-ons, and I'm sure more are coming. Will add-ons ever do everything mods do? I don't know, but I do know mods won't have some of the features of add-ons either.
Are we able to make new recipes/blocks/entities for Minecraft Windows 10 Edition? Or are we stuck only being able to edit what's already there?
However, there's already been one update with additions to the functionality of add-ons, and I'm sure more are coming. Will add-ons ever do everything mods do? I don't know, but I do know mods won't have some of the features of add-ons either.