For example I have a map on the back burner that is probably 70% done which is a cave map with lava. There is a cliff ledge providing a short rush distance between mains with destructable rocks above the ledge holding back a lava flow. If the destructable rocks are destroyed then lava is free to flow down over the ledge blocking this path (and killing any units on the ledge at the time.
This results in a fairly normal game of SC2 but provides some interesting possibilities:
If you see your opponent going for a rush strategy you can increase the rush distance on the map.
If you wish to play aggressively then it is in your interest to try and protect the rocks.
The base layout is such that it is akward to take a 3rd base with this path still active so you need to take out the rocks before going for your 3rd.
There are many other simple concepts such as this that do not have a dramatic effect on the gameplay but can make a map have a very unique feel to it and provide for an interesting game some examples are:
A map with a swamp that slows massive units
A city map with a monorail network where if you kill the monorail it will drop some resources (done this one already)
A map with lightning storms along a path by air which deal small amounts of constant damage thus detering light air units (mutas/pheonix) but being less of a deterant to large air units (battlecruisers/etc)
I'm not talking massive changes. Subtlety is the key word to remember here.
Obviously things like this are impossible to balance for tournament play but is there an interest for the sake of fun custom maps for things like this? Because I don't see anyone making these kind of maps.