The problem with an Attack on Titan themed MU is everyone wants to be the special snowflake who can turn into a Titan.
That or they want to be the uber bad ass chick who can kill anyone and anything.
The theme itself is rather bland, and this is true of /most/ actual anime, what hooks us is the characters, not the theme which is why there are so many anime tropes when it comes to characters and why we can pick them out so readily. It also makes them familiar which makes an Anime easy to get into because we have our favorite types of characters and we want to see how those characters make it out in this anime.
But the actual setting? Horrible. Most people are the equivalent to peasants, there's an unbeatable monster out there that is just destroying and killing everyone. So even if everyone was part of the military, it'd be a one hit kill type of system. They get you once, you're dead, which would be crazy high turnover and the real lack of story variety would be... well I don't know.
It's not to me, maybe it's just something I don't see surviving for long. I'm of the opinions there needs to be a plethora of enemies and situations so as to not succumb to burnout/over saturation.