Logically speaking, there were many choices. There could have been a blanket ban on slavery in any territory. Conversely, there could have been a rule that said that slavery would be legal in all territories. Congress could have picked (as it did in the Missouri Compromise) which territories could have slavery and which could not. Finally, Congress could have allowed some or all of the territories to decide for themselves whether they wanted to have slaves or not. This is Congress did with many territories in the Compromise of 1850 and with still more of them in the Kansas-Nebraska Act.