To be honest, I'm not going to let people write custom code in their trials, since that's more or less what Sing is suggesting.
Remember when henke37 was toying around with V5 (or V4, maybe), and wrote a trial that replaced your forum signature when you played it
? It was just a silly joke, but it could be much worse.
It was possible precisely because a flaw in the code at this time allowed him to introduce JS code to be executed.
What, however, I would be happy to do is to integrate new functionalities in the AAO engine even if they are not coming from AA games - provided these features have enough potential to be reused by many people.
Given that people have been using AAO for all sort of visual novels aside from the AA universe, it could make sense to include new features from other games, or even original features.
As for integrating such features as "modules" or "plugins" as Ganon suggests, the idea may be good if there is demand for many new functionalities in the end, and will certainly be considered, but it's would complexify the structure of the code quite a lot.
Currently, the code is mostly split between editor and player : the editor lets the user specify inputs for all features, and the player renders them all when needed.
What you suggest would require rearranging the code so that it's split by gameplay elements : each package handling both the editor interface and the player rendering engine for each functionality (eg CE, investigation, logic, etc.).
This sounds like some work for AAOv7