Plugin system should help with that as plugins code will run faster than LUA. There are limitations currently on the amounts of objects in the game (Such as Particles), though if they were ever hit then they could be made to be dynamic instead of a statically allocated amount(I am not changing them now as i don't have a test case which makes them hit there limits).As far as being an open ended game like freelancer or the X Series of space games we might be somewhat limited (performance wise).
Its proberly most similar to the Elder Scrolls.I'm sorry but I am a bit confused; when you say "Plugin System" what type of system are you going to implement; what do you mean when you say that? When I hear "Plugin System" I think of things like add-ons for The Sims Series or Mod plug-ins for the Elder Scrolls Series. Are you trying to develop a system where people can enable or disable a certain modification or feature?
At this stage i don't. I have up to this point only maintained Tarr Chronicles and Dark Horizon, fixing bugs in them, i have not tried to mod them, so i do not know there current limitations. Though when limitations popup i should be able to remove them.Do you know what things might be hard coded? For me it will be best if I start small and work my way up to harder things.