[Ghoulish Delight]

I'm sure it's a little of both. They may have had the broad strokes of the story arc in mind, however the details of how they were going to get from A to B had to at the very least have a lot of flexibility built in.

Among other things, it would have been lunacy to plan for a 5 year show (that was their original plan, stretched to 7 after it proved successful) and not have a way to deal with unexpected things like cast members leaving, popularity waning, unexpected popularity of what they thought would be a minor character.

The way I figure it, they had big picture things like, "The island is moving in time", "there was a society of scientists there beforehand", and "the island is a complex analogy for the greek concepts of hades and the river styx" or whatever it turns out being. But the details of who is working with whom, or against whom, or the mechanics behind how and why the island might be moving in time would by necessity had to have remained fluid.