Yes! It is possible to use HaxeFlixel as a framework for a 2D MMORPG. I've made a 2D MMORPG using HaxeFlixel and it's been great, I've never had any regrets about choosing HaxeFlixel and would use it again if I was starting a new 2D MMORPG project. If you'd like to check out my game as an example of an MMORPG made with HaxeFlixel, you can find it here: http://www.portalwalker.com