Another idea would be to add in a wildcard. This wildcard would be some predatorial derelict species escaped from who knows what, only making an appearance from time to time. Very dangerous and very deadly.
And/or, this wildcard could even be a secret government experiment gone wrong and has escaped captivity. Some huge mutated bear like from Prophecy(movie 1970's), or some creature created by combined strengths of other super predators. Again, only rarely making an appearance.
Adding mutagens into the game is always a plus and will keep your party guessing. Radioactive, biological, and chemical mutagens are all good challenges.