So, to understand the OP, you want to make a 2.5D isometric fantasy MMO? Well, there is no really good, complete engine for an MMO. Many come with major trade-offs.
There are three general avenues:
1) Flash-based: there are a few isometric engines out there for this kind of need. For example, look at OpenSpace to get an idea of what I am talking about.
2) Render-engine-plus-grunt-work: Grab some libraries/engines to headstart your work, and code the layer that sits above it all and coordinates it. The engine typically mentioned is FIFE; there are many others. Add a networking library and such, code the client, and you are off to the races.
3) MMO Kits: With an environment like Realmcrafter or Multiverse, you can get up and running faster, but many of those "kits" have questionable scalability usually due to poor coding.
PS: Hmm, looks like we have a Visual3D spam storm again. I was away for about two weeks and I am noticing a slew of newbie posters cheerleading Visual3D all of a sudden.
PPS: I love the "I recommend Visual3D for your MMO, even though networking sucks donkeys" statement. Would good, scalable networking be central to an MMO?