I´m already flying through levels with my 20 100% XP boosts from a pack I bought, I get those on each character!
Credits gain would only increase prices on the tradepost, but maybe not a bad suggestion
Rare drop chance increase would be cool
I´d also go with
A subscriber exclusive new lvl 1-20 starter area?
Subscriber exclusive mounts?
Free item of the month (see TSW)?
Early access on new content (see TSW)?