subarashi

The coolest thing you can do with your maps when it comes to slime or other liquids is connecting them in some way through streams or falls. I really dig attention to detail like that.

I say most things are fair game when it comes to secrets. This was mentioned before above, but my biggest pet peeve are secrets that are mandatory for progression. Don't do that. Personally, I believe that following what the commercial releases do a lot of the time (particularly Knee Deep in the Dead) will give you a good idea as to how secrets can help the player whilst also not halting important things like natural progression and flow of a map.