Say you've found an RPG you love, and you want to create stuff for it. What is important for that game to have, modding-wise?
Is accessibility most important: that it come with easy-to-use modding tools? Or does that matter less than having extremely broad modding capability that's not necessarily assisted by tools? Do you mainly want the ability to make changes to the existing game; or is the ability to create and distribute original adventures more important? Do you care about the ability to create and distribute stand-alone assets (like character sprites, portraits, or music) that can be used by any adventure, or are you mainly interested in distributing original assets as part of a new adventure? Do you care about the ability to modify AI?
In short: what do you consider top priority?
Look closely at the Legend of Grimrock games. These came out seven & five years ago respectively. The original game did not have modding tools, but during development they created an integrated level editor for the first game, and added it in [after release] in an early engine patch. This is the single reason that their games are actively modded to this day.
They included a level editor that allows a complete novice to create a working starter map in minutes. They can test it out in the editor, and/or export the (self contained .dat) map file to play it in the regular game, or distribute it to others, so they can play the custom adventure. The editor uses visual connectors between objects; a lever connects to trap-door, and opens or closes it when triggered in the game. User scripts [objects] allow for more complicated behaviors, and to forgo the need for the visual connections once they are past the basics.
The community (one fellow in particular) wrote a model importer based on their published file structures. Later (with help) this was expanded to include support for object & skinned animations. Another fellow made a full Blender import/export plugin that allows modders to import / export working model & animation assets.
Grimrock 2 greatly expanded the scripting capabilities, and made (on the fly) customizing of objects (including monsters) a possibility through user scripting—no more need for fully defined object copies, to make changes, as in the first game. This does mean that the modder can fully re-write the monster AI used in the game by any and all enemies; though the game does provide several specialty AI brains for the monsters. It is also possible to just override the default brain for conditional behavior; letting the default brain handle the rest.
*This expanded scripting model was a double edged sword for them, though. It was in many ways not compatible with the earlier, simpler version that most people (who did learn) had learned, and in a way that was not obvious, and broke working scripts from the first game.
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So... The key points I look for are easy basic mapping, user scripting, model, sound, video, and animation import/export possibilities—if not full support for it, and of course... entity editing, to allow for monster customization; [hitpoints, string data, attacks, special attacks and other abilities etc...].
I'd also vote for other scripting languages, like Python or Lua, the latter, I've heard, is great for games.
Grimrock 1 & 2 use Lua [5.1], but the game does restrict [omit] support for certain (most?) os functions. Mods are intended to only interact with the internal game, not the OS platform. This does make the limitation that Grimrock mods are not intended to make use of loose files; and thus user mod patch releases must be a re-export of the entire mod.