My idea is something like this
Every 500 kills, your rewards improve by one tier.
Tier 1 : 2.5 gold, 10 WvW exclusive bags
Tier 2: 5 gold, 20 WvW exclusive bags
Tier 3: 8 gold, 50 WvW exclusive bags
and so on.
The bags can contain tokens that can redeem WvW exclusive skins or such that can be obtained through this.
This way you encourage players to actively participate in defending or following commander, instead of repairing a structure for rewards, which only encourage afk players and troll players.