It's great when you have 3 players for a 2 player game, so 1 Player can referee the game and throw in unexpected events, plot twists, and surprises.
See my sheets for designing refereed scenarios
But when you don't have a referee, you can use Event Cards. These cards will allow players to cause surprises and changes to happen in the middle of a battle.
Event Cards Download
The idea comes from an old Necromunda supplement by Andy Chambers, Jervis Johnson, and David Moretti.
Every time we play, we invent another 2 or 3. The current count is 30 - our goal is 52 (so you can use a regular deck of cards and print on label paper).