I agree with darknex, there should be some sort of editor at the begining of he ame where you can create your own creature, or chose from a group of premade creatures...
but I think it should go a step further than spore, and you should be able o add fur if you want to...
Then Gary has slime legs and arms. And the Octopus can't go on land unless there's underwater creatures. If there is, I'd choose Whale. Yes it has arms(and legs), but i don't know where.
A sort of plant creature. Maybe a man eating Venus fly trap? It would have roots as legs and its arms would be the part of the stems that extend out where the leaves attach to. A tree would be a nice addition too.
Next to that... I'd want a wurm-like creature that would move around by burrowing underground. It could be attacked (when it's underground the animation is just dirt covering the animal so you could shoot at it or melee it) but it would be harder to kill. The wurm creature would be substantially weaker though, because it has the burrowing advantage.
Finally, I'd want them to keep all the original creatures. I want the cow to be turned into a bull though, because a bull seems more appropriate for a creature who's gonna be working in the fields and fighting. Cows are only good for entertaining. Maybe add bulls as a new one and keep the cows?