I really hope there will be one. If there isn't, then we'll have to make our own, or hack some existing tool, like Bethesda's Geck, to do the job. Either way, it'll just never be as good, and will be extremely frustrating to work with.
It's interesting to compare single-player sandbox games in their treatment of modders.
Rockstar largely ignores them: its GTA4 forums ("social club") doesn't mention or feature the modding community. The only mods available are cheats and cosmetics: new skins, sounds, and vehicle/weapon meshes. Installing these mods is complicated and dangerous for new users. No new content.
Bethesda embraces them, highlighting the best ones on its blog, giving them sections of its forums, providing editing tools, and as a result has what is without a doubt the best modding community on the planet, which gives them many advantages; a sizeably sticky fanbase of modders; a larger pool of fans who play the game *for the mods*; hence a market of people who are willing to download their "DLC" mods, providing another income stream from the game; a ready and willing pool of ready-trained designers with sizeable public portfolios, who are experienced in Bethesda's own in-house tools; and much more. Mods add new characters, plot, areas, animations, scripts and so forth, as well as huge amounts of the kinds of cosmetic changes that GTA gets. Tons of new content adding many hours more play. Installing these mods is a 1-click operation.
It's a shame for GTA4, since there is so much scope in that engine for more stuff to be done. But it's a time tradeoff: do people spend a week reverse engineering the texture format, and how they are indexed and referenced, and writing a program to reinsert them, all to change one texture of one NPC... Yes, people do that, heck I helped write DEU back in the days of Doom, and before that, hacked Underworld's item lists, map formats, and savegames.
But in the same time it takes to reverse engineer a change to one texture, we could spend the same time in Fallout creating a whole new NPC, with a house, backstory, custom AI and daily habits, inventory, wardrobe, conversation tree, quests, his...
So the engine with the ready-built editing tool wins out. When I look at GTA, I just see a collection of animation data that might be nice to extract at some point, but nothing else really appealing. With Fallout3, I see somewhere in which I can create worlds.
I'm really hoping Divinity II will be on the friendly side of the fence. I'm itching to play with it, even more than to play it!
