I was wondering if there's any kind of advice you would give to someone who has also been trying to come up with a whole original world? Mostly concerning creature designs, as that seems to really be the one thing I have a lot of trouble with. It's like that creative part of my brain shuts down when I'm trying to figure out something as simple as creating a more interesting species of cats or something.

Well, having themes to work with can help a lot.  If you’re just sort of trying to make things in the dark, you’re gonna not have anything that’s… cohesive. Even if you end up tossing all of your first ideas (as you will undoubtedly do) there will be some interesting pieces you can take from it as you start to figure out themes that matter to you. 

For instance… there is an overarching theme of plants + animals combined, and of genetic anomalies in Flora (what I’m working on) which helps all of the designs sort of feel like they belong together in the same place.  That cohesiveness.

So anyway, you need to just make some sheets of drawings even if you feel like you have no ideas.  You probably won’t at first, but you have to get through those to find out where your natural inclinations take you!