Not that I am speaking for everyone, but I think that what we are asking for shouldn't be that difficult from game developers such as yourselves that made Sins in the first place.
Actually it would be. Currently, everything we can do in Sins is modular, basically plug and play different pieces that are independent of each other. You can make a particle and simply plug it in an entity, or a mesh. But if something is hardcoded into the engine, like entity types, you can no longer simply make a different one and have it work. If each entity type had its own external definition file that we could modify, we could easily just make a new one and the game would read it as it does with entities themselves and know what to do with it, because it would maintain its modular design. But as it is, this kind of thing doesn't exist. Engine limitations are usually always there for a reason, and the code expects those limitations in order to work. Allowing us to "add" completely new entity types, buffs, research would probably mean that IC has to make them accessible externally, in essence making them "modules" as all the existing stuff.
And as psugar said, this basically would mean having to re-write the engine. But then we would still be limited to what we could put into them, because without access to the source code we can't teach the engine to read things it doesn't know how to. For example if we wanted to make another "resource" for strike craft, other than command points, sure we could add "PilotCrews" to the entity files, but how would we teach the engine what to do with it? That would require a more open engine and basically an SDK for us, the ability to script and modify the engine itself - which it isn't designed for right now.
I mean, it'd be great if eventually we gained some limited ability to script in new things, but it is a lot of work for them, too.