Every DM's secret weapon

Shhhhhhhhhhh

If I don't know my master plan, then THEY certainly don't either!

Also I do intentionally leave some things a bit more vague, with ideas of what I want to do, but I'm hoping the players will start theorizing amongst themselves so I can incorporate their stuff into the game.

It gives them a nice sense of accomplishment to have "predicted" something, and I think they feel more engaged in the campaign.

Or maybe I'm just a bad DM...