Look for the source code for L2Editor, and if you have enough knowledge and time, you could create additional tools for it, resulting in a really comprehensive package, since it’s missing a lot of features…
Although personally, if I knew how, I’d make a new version of L2Editor that’s much more stable and has a more user-friendly interface, without any lag—but since I don’t know anything about that, I’d rather wait for someone else to step up xD