Disclaimer: I've never done this sort of work professionally, nor worked on vague themes. (That weren't my own vague themes).
But typically what I do is create a document in the screen resolution with the palette I'm using. I then quickly sketch photos that match the theme in this document, to see what will read, what colors work together to create objects etc.
Here's an example of things from a "beach" theme:
I vaguely keep the restrictions (if any) in mind while drawing, but typically just want to know what will read at the given size/resolution/palette. From there I take what's interesting and put it into the set, which is where things get imaginative. The middle picture is a different kind of beach than the bottom right, but you can figure out how to connect them in your set to create "unique" structures. The top left is actually already a combination of ideas. The original image was pretty plain, so I ran through my random slideshow to find other things to add to it.
I don't have much info for finding good ref, I just save cool images while browsing in general so they tend to be there when I need them. I put what I have on a random slideshow and draw what comes up. Sometimes I have to hunt a little, and I do what you say. Look for search terms related to theme. And this can be tedious, but it also adds to collection for whenever you might next have to draw a similar thing.
Also, sometimes looking at textures (from/for 3D games) can be really interesting. Because they're built to tile/loop obviously, but I didn't think of them until I encountered some in another search.
For how to convert things to slopes/such... typically while sketching it just comes to me. "This would be a good slope. This could be a drop-through platform."
I have no advice on the perspective issue. The cove in the bottom middle? Realized I couldn't use it after actually making real tiles out of it, because it implies there's water back there, and it not being horizontally aligned with water when it ends is... weird. Hopefully that's helpful.
Edit: I'm reminded of this:
http://www.pixeljoint.com/pixelart/87096.htmThat dude drew 100 trees from reference. I looked at this and checked out every tree in his ref list that looked cool. That Rainbow eucalyptus, mmm.... I saved images of every single tree I thought was cool. Even though... I'm not working on trees right now. So next time I'm to make a forest set, or trees I'll have REALLY COOL INSPIRING images already. Stuff I wanted to save just because I liked it so much, not because it was for a project. This is also how I attempt to avoid generic stuff. if I need to search while I'm making the thing, I'll tend to settle for less cool and less unique images. My goal is always to preempt a problem of inspiration. If I see things like, "Places on earth you won't believe are real." I'll look at it and save the cool stuff.