Long ago, in the before times, my money was stolen away from me by a series called .hack, and my time by a game called Phantasy Star Online. A futuristic setting, unique classes and environments, central hub towns and portal-styled instances dungeons, and of course, lots of loot and grinding. The closest thing I've ever found on iOS is Mission Europa...but unfortunately, I just got a 5S and haven't had a device since Gen 2 iPod Touch, and ME doesn't work on my 5S. Help me dig a little deeper here, guys. I'm not even making multiplayer a requirement.
It's ship combat, but Galaxy on Fire 2 HD is probably your best bet. Mass Effect works fine on my 5S FYI, but it's not as deep as it's console brethren. Order and Chaos online is a fantasy MMO (but yeah, who cares about fantasy amirite?) Hunters 2 is a TBS with classes. Heheh. Holy cow I'm dumb. Knights of the Old Republic of course. Go buy it. Now.
Now that i've been reminded of it, i think we need a .hack port. One of the most unique RPG experiences i've had. Played and completed all of them. Left me very satisfied.