I would like to see increasing difficulties for repeat shots, or decreasing difficulties for finding a sniper who is hanging out shooting at you. It'd also be neat if snipers may spend even more time between each shot to make sure they aren't spotted.
Example: Bob the Sniper is lying in a bush picking off travelers. Round 1, Bob shoots at traveler, with his hide check on 6, opposed by an 5 detection (whatever skill this is) check. The higher percentile "wins" the contest. If Bob shoots again on round 2, he gets +3 difficulty, if he shoots again on round 3, he gets +2 difficulty, if he shoots again on round 4, he gets +1 difficulty, and on round 5 he'll simply check again at 6. During all these rounds, the traveler may continue rolling detection each round to find Bob, but Bob may not roll stealth again until after he has taken another action (such as running around and hiding). Bob must sit still and be patient, taking no action for one round, to decrease the difficulty penalty. E.g. if Bob gets up and moves on round 2 in order to another position (in order to re-roll Stealth and try to get a higher number), he will continue to be at +3 difficulty to snipe.
This is just an idea. I think it works well, because it makes sniping very difficult and requires patience, much like real sniping. Plus, they are (theoretically) taking a huge minus to their +dodge, if they are even allowed to +dodge, and so you're doing a ton of extra damage.
Edit: A better way of thinking of this system is that you get +3 difficulty to each subsequent shot. You may decrease the difficulty penalty by one for each round you spend doing nothing but aiming.