I'm working with my poor, homely modding skill on a mine variant right now. It will be the same basic principle for all factions. It sits in its place until an enemy unit enters the grav well. All mines would be placed by minelaying ships so they can be used anywhere.
If it's within range it launches at the enemy at very high speed and from greater ranges than current mines and explodes for a single shot of much greater damage. I know how to make a "frigate" do this but my goal is to do it with mines.
In order to make mines do what they do in real life--intimidate enemies from passing over them, I'll have researchable mines so they increase with effect as research increases.
If I can swing it I'll add later mines that produce emp style effects,anti-shield/hull/ability mines and dummy mines that are cheap and do almost nothing but serve as deterrents along with a special one I ain't gonna discuss out loud right now. 
Some mines will also be tailored to hit specific ship types.
My ideas are based on modern mines which are incredibly varied and sophisticated and don't have analogous representatives in the game.
Mines will max out at 10 per grav well--enough for variety and strategy but small enough not to overkill. You will have to actually think about placement strategy.
Mine fields as they exist in real life and as they are modeled in the game are completely unrealistic. The sheer volume of space that would need to be covered is really astronomical and would bankrupt economies just maintaining and replacing them.
Now that I have the black hole technology, nothing is impossible. Muwahahahahahah!
