One of the challenges with multiplayer games on mobile platforms is that data networks offer wildly different speeds depending on your provider and location. A great way to get around that is to make your game card-based.
You likely don't need me to tell you what makes Hearthstone so great, but in case you missed the boat on it when it came out, Hearthstone is a card-based game, slightly similar to Magic: The Gathering, but much simpler. It's easy to pick up but hard to master. Once you do, however, it's incredibly satisfying to cut a swath through your friends' decks and demolish your foes online.
Now, of course, paying for the in-app purchases nets you better cards, sure, but many classes don't really need those types of cards to succeed, relying instead on combos and sly tricks with common cards. Premium cards are never absolutely necessary, and that's a nice touch.