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Well, I've read all the ideas... can I just say what I like and what I don't? The idea for the 3rd level is very interesting (although it's difficult to implement). I'm all for it! I liked the idea of training for diving underwater/ascending. The only BUT is that not all creatures appealed to me... for some reason, I didn't really like the idea of crocodiles (although biting is cool). Also, I don't think turtles are a good fit, IMHO. The main reason for this attitude is that in Heroes, units are more mythological creatures than real ones... you can still tie a turtle to myths, but a crocodile? Somehow, it doesn't work...
P.S. But please don't be offended by my opinion... I just said what I think...