I can only tell from what I heard.

The Enging (Unity) doesn't like it to be modified. It was planed to make this possible in the really early kickstarter times, but it was far more complicated.
What you can do is exchanging images.

If you want to have mutators for the map editor, they are planing to add some, but dunno when.

That's too bad. I think i had some good ideas.

Like (nearly) impenetrable walls so you have to focus more on dungeon design.
For example a dungeon core which can support itself while an Invasion, would be necessary, or at least an important strategic advantage.

At the ,moment you have to double and triple build everything, because an underminer just vanishes every logical form you give your dungeon. (or 20 opposing Imps, running around)

Also adjusting the needs and makeing the creatures more valuable.
This would lead to an extended dungeon life, where it would have strong disadvantages to loose
your servants. (leveling your creatures would be more important)

In general my plan was to influence the game by Tower defence like and Stronghold game elements.

In my opinion that would be a huge opportunity, because in my opinion the game itself is really good, but does not even benefit from half of its potential.