Im planning on reworking Italys cancer focus tree by adding some historical paths like and adding ahistorical or fun paths.

As long as it's fun, that's all that matters.

As you will see when looking at base-game focus trees, there's usually 2 general approaches:

1. Everything is a prerequisite of an initial choice (Think Spain's focus tree)
2. There's specific branches for specific things (Think the generic focus tree)

Option 1 allows you to force the player down a very specific path; for example, not focusing solely on industry for 3 years before buffing their military. But doing this may lead to situations where the player is unable to respond to situation in the world if, when creating the tree, you time everything to be available at almost the perfect moment. For example, if you had a focus which would denounce Germany and would be next in line to start the time the Anschluss should have happened (on historical AI), the entire tree may be useless if Germany decided to have a civil war/delayed Anschluss for some reason.

Option 2 allows the player to choose how they want to play a bit more. If they wanted to focus on the navy first before touching the army, then that option is there for them. Or if they wanted to build up for a big fight early on, then they can take industrial focuses earlier.