I am thinking about buying a game bundle on steam, and I know the game developer is still going to add more stuff to the bundle, like their new DLC's. Do I need to re-buy the bundle, or do I get all the content that gets added anyway?
No. Bundles are for getting a collection of a specific set of games and/or DLC's. Some games with a lot of DLC's have multiple bundle options with different sets of DLC's (often including the main game). If content is added later on, the bundle would need to be updated - which often happens. A good example if the publisher bundles where you can buy all the games and DLC's of a certain publisher. When new games are released, they are usually directly available in the bundle as well.
A bundle is also a special type of collection, where you only pay for the content you do not yet have. However, sometimes it is actually cheaper if you have most of the content already, so make sure to double check. So buying the bundle again later on, might give you a discount on the new released content. In my experience the bundle discount is usually around 10%.
The version where the DLC's would be included are often called a 'pass', like Season pass. These passes usually allow you access to all current and future content of the game the pass is meant for.