In theory there could be features that are based on subscription.
These features could be anything from more developed and advanced options
to excisting features, up to totaly new features.
One new feature could be a racingsystem for vehicles that is based on the
instance system. To enter and race in instance you need to pay
a monthly fee, but to trade and create parts, vehicles and accessories you have access
to it as any other item in EU, in any area in EU.
Another alternative could be a pvp combat school, where you pay monthly fee
to enter a pvp instance and can use all equipment as much as you want without
extra cost. EUs own lasergame in a way...
As with everything, when we get something for free or much cheaper, we have
to lose something else, so no ordinary skillgains here.
What you do get is RL skill to fight in pvp in EU.
If any of these or other ideas work in reality, that I'm not sure of.