Minefield is a good idea. A similar concept has been implemented in SMACX, but I never bought it (demo keep crashing on my system, so I stayed with my SMAC).
About your proposal, my opinions are the following.
- mine lasting xx turn: agreed (exact number depending by game balance)
- mines laided down by appropriate unit: agreed, but engineers are good enough for me, but because a ship or submarine equivalent is needed for doing the same at sea, maybe a special added by Unit workshop will be goo enough. BTW German subs did a lot of mining operation during wwI & II.
- special unit able to see mines: for sake of simplicity, they should be the same able to laid mine
- mine effect reduced against tanks: keep it simple, let this out; mine already end effects after xx turn, new mines will be more powerful by default.
- mines affected submarines instead, still I agree not planes (in WW2 there used to be balloons over ships and cities, to have similar effects, but without great success AFAIK)
- Mines belong to someone, to be true. If you laied down a minefield you must have a map to help your troops to pass minefield without harm. Your minefield should stay visible on your map.
- removing of minefields should be more a part of peace treaty, IMHO. If someone accept to remove mines, the normal rate of mine "decay" will double (e.g. each turn have a 66% chance of losing a HP), not instant removal because mines are difficult to find and disarm.
- UN can properly vote to ban use of mines because considered atrocities, but nations can refuse to approve this (USA and China do it actually, AFAIK).
All things considered, yours a very good mine model, UltraSonix.
BTW UltraSonix, looking at your signature I must ask you now: where in the neck do australians keep kangaroos and koalas, then?
------------------
Admiral Naismith AKA mcostant