I think skirmish is a lot more interesting and challenging when you're always moving, but from what I remember (I haven't played in ages) there isn't too much reason not to just bunker down a hole with a sniper rifle.

Anyways my idea is to have a line of death moving across the map. You have to keep moving because if you touch it you die. Maybe it can accelerate over time or something. I dunno I just had this idea in my head for ages and I wanted to clear some space.

If you had lasers falling from the sky it would look better.

Or maybe having a bunker buster missile that would target you if you don't move from a certain area around you for some time. Note that the area would move with you once you get out of it.

Wall of fire and procedurally generated map.

Now you just have to do the procedurally generated map bit.