This is not a term I use lightly. Sven was a creepy bugger if anything, and many of his ideas were almost outrageous, but there is one thing of his at least that was genious: The Story. The story he wrote for his hoax was perfect. It laid out exactly what the multicellular stage MUST have, and here I will reiterate them.
1. Gameplay MUST Effect Your Creature's Biology
When playing the game, your actions therein must effect what you creature ends up being. Sven's worm story lays this out perfectly. It takes many generations before he crawls onto land, but the fact that he's trying to get onto land is why he does. When he developes a habit of digging in the soil to find food, his worm grows a specialized jaw to handle this. In Thrive, this must be able to happen. Spore suffered from a complete lack of continuity between editor and game. The only effect your gameplay had on the editor was discovering parts, and actions in the cell stage effected diet in the creature stage. In Thrive, we have the opportunity to make a truely wonderfull mesh of gameplay vs. editor.
2. Sentience/Sapience is Earned Through Actions That Require Intelligence to Perform
Sven's worm, while still aquatic, would lose intelligence, or at least grow stagnent, if it used camoflauge to avoid predators, or run away from them. It only developed through exploration, and manipulating the world around it. If he hunted, the worm got smarter. Playing with sticks to drive off predators gave massive bonuses. Curiosity would be dangerous, as there is always a chance of it getting him killed, but rewarded him with a chance to go further into the game. Thrive must similarly do away with Spore's "Do X to win" and replace it with a realistic and far more interesting style of play that rewards exploration, experimentation, and good thinking, and punishes repitition and grind.
3. The World Must Feel Coherent With The Actions Taking Place Within It
When playing, Sven chose not to customize his world and instead let him discover it as he went along. While playing he slowly learned it was high-gravity due to a lack of flying creatures, squat life, and a good deal for experimentation with fruit. He also noticed an abundance of coastline, and a large number of semi-aquatic organisms. This painted a picture of a planet. In Spore, all planets were the same. Everything could survive in all environments. This meant that the only thing taken into account when chosing a world was appearance. Thrive must have a connect between world, inhabitants, and possible actions on the world.