I recommend the MMH55.5 mod. It's excellent, very interesting, and quite extensive. The instructions explain how to patch it with NCF, which will give you new creatures.
Overall, you can find a topic nearby with a detailed description.
I myself played Eve for a very long time, but Eve is a well-developed collection, while 5.5 is an independent mod, even though it includes already known elements.
I don't like the class system in 5.5; because of it, you can't add any other heroes besides those that are already available.
