What I don't need is an MMORPG engine, at the moment.

What I do need is a flexible easy-to-use engine that I can make a mock-up with. I don't need support for more than 10 players in an instance, so any multiplayer platform is probably fine.

I need an engine with which I can create the following core features:

• Waves of simple AI enemies that have specific objectives (move to point A, destroy target, move to point B). The units present can be between 50-200 in number.
• An over-the-shoulder view and the ability to control a team of 3 (like Mass Effect or the latest Dragon Age)
• Functioning inventory system

Right now, all I can really think of is Unreal or Source. Any other suggestions? Again, this is a proving mock-up, not an actual MMO.

I'm not terribly worried about the visual aspects as we just want to test mechanics.

Note: Can write some scripts in Python, Ruby, or Lua, if necessary.