Considering that I haven't made
anything for either RTW or WA in waaaaay too long (university, lack of time, yadda yadda) and when I
do make something, it tends to be excessively difficult, I shouldn't be the one talking about this, but if my history of RTW designs is good enough for this...
... try to think of all kinds of weird things you can do. Try to think of object interactions that are known, but rarely used, or situations that mislead the player somehow. Make the solution dependent on something unusual. Then disguise the solution.
A few examples of potential "unusual"s here:
1. Turtles can temporarily block scritters, allowing you to (for example) run through a gate before the scritter walks off the button. If the turtle is somewhere on the other half of the level, this won't be too obvious.
2. Scritters cannot pass through coins/gems/keys, but you can. I think I already used this one, but good tricks can be used more than once.

I didn't put real misdirection into that level, but I think with misdirection this can make a puzzle look totally impossible.
3. Grow+Ice=Growsicle. Growsicle+Spellball=Doing Something Offscreen. (careful: don't abuse weird angles here, otherwise this will prove frustrating)
4. Flashing coins/gems/keys and reflashing them can be an unobvious way to protect yourself from enemies. Careful usage of this can make tricky puzzles.
5. Try to design a magic puzzle (brr, for example) that has a simple, logical, obvious solution in 9 charges... but can be solved in 8 with an extreme amount of work and tricksiness. Then use the remaining brr charge in the other half of the level in some way that won't be too obvious.
6. ... more to come? Use these as a trampoline for your own tricks and ideas. Just think about random combinations of objects, most of them will be useless, but some can (and will) give you ideas that you've never imagined before. Go on... you can do it!
What do you think of a general-idea-based approach? Lately I was planning the levels for one of my hub areas and I though it would be nice to have a FlipBridge + Crab + Stinker Rescue level, Floing + Water Chomper + Get all Gems one, Blink + DragonTurtle + Conveyor adventure etc. Is it helpful or limiting for you?
It might work well, but if something doesn't work out, try to switch to a different general idea. This often works.