I'm in favor of distancing Ember from the MMO genre. Whenever I hear "MMO", I think of time sinks, grinding, and not being able to have real fun until endgame. Being forced into groups to get any sort of progress is also a perceived drawback.
The terms MMO(RPG), RTS, FPS, MOBA, etc. each describe a style of game and associated gameplay. Ember looks to be a combination of two or more existing styles. There have been a few games that did that sort of thing already, but they sometimes mash together various existing abbreviations in an attempt to describe themselves, if they don't completely adopt one of the simpler terms.
Let's think about what we expect to see in Ember, and then try to make a short, easy abbreviation if possible. Here are a few considerations:
Multiplayer vs Online: Multiplayer implies online, but online doesn't necessarily imply multiplayer. Since it doesn't look like you'll be able to avoid strangers completely, going with Multiplayer would be most appropriate.
Shooter: This describes the interface, gameplay, and pacing one can expect when playing the game. We won't need to differentiate between first and third person perspectives because it will have both.
Arena vs Battlefield vs Open: This tells you the scale of the game world you can expect to inhabit. An arena would be on the level of MOBAs and classic FPS. Battlefields are like beefed-up arenas, in that they are still finite, but of course are much larger. An open world would imply a grand scale, like a continent, planet, or even galaxy. I think there's a better word than "open" to describe something like that, but I can't think of it right now.
