Honestly, I think a lot of it is limitations of their engine, which I feel is really bad. I think this is why we haven't gotten entirely new activities. It can only do a few things and those have been since the beginning:

* watch a cutscene
* kill an enemy
* FATEs
* click on a thing to increase a counter or trigger next stage
* the camera zoom thing
* talk to NPCs at specific points
* use a special action
* act as a character with a special move set (usually once per patch at most)

That's a really short list and some of those were the only new kind of activity in an entire expansion. It's partly why I think Island Sanctuary was so poor -- their engine didn't really let them do anything interesting.

The idea that you would get a game where you chase someone, or bombing runs, or taking over a super powerful vehicle or entity and laying waste to things seems impossible. Or even just mini games in the overworld or hunting for treasure chests that randomly spawn. Honestly, I find the overworld painfully dull, even visually most of the time. It's partly why everyone's in the cities. Why on earth would you do anything in the overworld unless forced.