Here is my ideas on the matter...
You walk down a destroyed pre-war road exhausted after a day of skirmishes. The raider gang 2 miles back took all of your assault rifle ammo to extinguish. The Super-Mutant ambush back in the abandoned warehouse used every shell you had for your shotgun. The only ammo remaining in your inventory are pistol rounds and those number 25; good for a hit-and-run. Your PA is degraded and heavily damaged, so you removed your helmet to get some fresh air. "Things will get better" you think to yourself.
Suddenly, you hear a heavy buzz behind you. Your heart rate increases tenfold, you think erratically for you know your life depends on your next move. You decide to jump behind a large rock, further hidden by the shade of medium-size tree. As the buzz gets closer you peer over the side of the rock to see what is approaching, and what you see makes your legs tremble. A frighteningly large Cazador, 4 times the size of a normal one, hovers along the road. Darting back behind the rock, unholster your pistol, and prepare for the worst. The Cazador stops abruptly (hearing the sound of your unholster), and begins to walk slowly towards the rock you hide behind. Sweating profoundly and hearing you doom approach, you slide the pistol carefully to the side of your head, close your eyes, and take a deep breath. Right before you decide to end it all, the Cazador suddenly darts back to the road and hovers away, continuing its unknown journey.
As the buzz fades away in the distance, you holster your pistol and breath a sigh of relief. It seems this was not your time. You put your helmet back on, walk to the road, and continue your journey. Hoping secretly that you never encounter that bug again.
Something along this line would be awesome, doesn't have to be a Cazador though. Could be a SM Behemoth, a legendary Bloatfly type creature, hostile PA patrol, 8-16 heavily armed and chemed-out raiders, F3 status Feral Ghoul Reaver, or anything really dangerous.
However, if unprepared/scared there should be the opt. to avoid them entirely and live another day.
Your thoughts?