Early days, but I say that every weapon/armour upgrade (if they exist) should only be available for purchase via game grind.
Then have a bunch of cosmetic items such as helmets, skins, armour parts, different "jet" colours, voice packs that can be purchased with real game money.
Throw in a deal like spend more than $10, get a permanent booster status, all exp earned toward item progression x1.5
Just my 2 cents.
Oh, and also, every Legions veteran gets an exclusive in-game flair or icon, or armour piece, or whatnot for our unfaltering dedication. THE LEGIONNAIRES ARE IMMORTAL.