These days it seems like the decision isn't "Can I build my game?", it's "How am I going to pick from all of the fantastic tools available to me?"

The OpenSpace Engine is a great looking toolset for developing isometric Flash MMO games.  It is still in a pre-release state right now, but everything I can see about it looks great.

It includes the tried and tested SmartFoxServer, customizable aspect ratio, scrolling, avatar management, and advanced pathfinding.  There is no word on price yet, but the demo they have online is impressive, as are some of the screenshots of the worlds currently being developed by their partners.