Thankfully, you won’t have to pay extra to get something. Weapon skins, world events, challenges and new rides (the Mech you see above, plus the One-Wheeler) are all slated to be available throughout 2019. You’ll have a reason to keep playing even if you blaze through main story, then. The question is whether or not Rage 2 will receive similar treatment once 2020 rolls around. It’s certainly nothing new for single-player games to get expansions, but they rarely have roadmaps with month-by-month commitments. We wouldn’t rule out a change in pace based on demand.
