One of the meta-stretch goals will depend on the number of backers. "From now on, for every 1000 backers that we gain, the lair of the monster beneath Phantom Forest will become deeper and deeper. (This means, we start counting at 10'000 backers.)" according to Larian. The game already has 10,980 backers as of this writing.
The next goal at $650K will add personality traits to PCs, with Talents that will enhance the PCs dialogue with NPCs. At $800K, henchmen graduate into full-blown companions with their own personality, history and goals. At $1M, Larian will add day/night cycles and weather systems, all of which affect NPCs, monsters and magic as well.
Gamers interested in backing Divinity: Original Sin can visit the Kickstarter page here.