This is more of a far future request, once both editor and player outgrow RM2k3 support and become a better version of it, but once that happens, it would be nice to see scripting support with the Lua language. Not the way XP and onwards did it (by rewriting basically the entire engine in Ruby, bad idea - some stuff (like event interpreter) need to remain in C/C++ for performance reasons, as well as any drawing code), but on top of it.
Basically being able to create small lua files that would implement custom commands and would have a powerful API behind it. The basic commands would still be processed internally by the C/C++ event interpreter (though with an ability to override the internal command with a lua script implementing a custom version of it) for the performance reasons, but this would allow for easy expansion of the engine in a similar way patches do, but much easier. This is not a new idea, mind you. Cherry did it as a patch for the 2k3 version (the one before 2k3 was on steam) before he created DynRPG, it was called LGSS (Lua Game Scripting System, a play on RGSS from XP and onwards).
Additionally, scripting of the editor (also in Lua) could be added as well, making it possible to create e.g. configuration windows for the custom Lua event commands.