Wait if You Want to Know What Else is Coming
We know quite a bit about the upcoming Star Wars Battlefront DLC but we don't know everything. And that might be a reason to wait.
Some of DICE's expansion packs for its previous games (Hardline, Battlefield 4) were excellent. Others left something to be desired.
If you are worried about paying for something you might not like or use, you should hold off until you know more about what is coming in Death Star, and the mysterious Battlefront expansion coming sometime early next year.
Again, you can buy these expansions one-by-one so if you're afraid of the unknown or if you simply can't see yourself playing Battlefront for the next year, you might want to save your money and skip the Season Pass. For now at least.