Inspired by the Tau from Warhammer 40k I thought this might be a good ability for a sniper to have. If you had two, one could be a spotter allowing the second to take full advantage of this ability as well as the rest of the team. This is intended for sci-fi settings mainly.
Paint the target
Marksman cost: 4
Instead of doing damage the sniper instead paints (marks) the target. This mark is compatible with smart weapons and advanced targeting systems. Once marked the target stays marked until the end of the snipers next turn.
Marked targets get no AC benefit from light cover or cover from obstructed view. And shooters get no penalties for low light. If the marked target is in plain sight attacks gain advantage. This applies to all attacks that can take advantage of the snipers mark.