We can have sticky thread here or something first. When you don't have editor or import/export tools, the only thing would be modding ideas.
I would like to see some sort of robust and easy to change file formats, not necessary text based.
Good game scripting engine and if possible game SDK.
And solid support for import/export assets( that's why file format is important, the easier to do it, be better people can come up with their own import/export plugin for their asset authoring tools.)
Adding new effects and spells/skills would be totally awesome than just modify existing ones.
