Definitions for mines:

the mines will have three types: infantry (antipersonnel Mine or AP Mine), for tanks (Antitank Mine or AT Mine) and for naval units (Naval Mine).

The engineers will have a X time to arm them, and they will be marked by a thick line between the provinces of vibrant hue (red for example), the extent will be limited but will be enough for a level 3 artillery to have the range on the other side, but as soon as the mine field is armed it will be a double-edged sword, if you or another opponent passes through it then the division in question will begin to lose ...

Now on the efficiency of the mine in different terrain, it is good to conclude with much consideration that anti-tank mines are efficient in forest land and hills, and for antipersonnel mines, flat land, mines will have a 25% bonus at these specified locations.

Now about naval mines: there may be two types of naval mines, one that is efficient against submarine destroyers and ships of trains and one against cruisers and stranded, something like for example Light naval mine and heavy naval mine ...

The naval mines will be armed by the engineers, but in the sea they will be a special vessel called a mining ship, it is advisable that they be escorted as they will have the same damage of a ship of train and also there will be a time X to arm them.

With World War II technology, naval mines could be placed anywhere in the Ocean, but it depends on your suggestion that the mine has a coastline limit so that it can be placed so that it carries out defensive and offensive functions

Detection of mines:

Landmines can be detected by other engineers and they can be disarmed at an X time.

Since the naval mines, there must be a separate ship in naval technology research, a ship called "minesweepers", they can detect and remove mines with an X time.

And every time a mined field is naval or terrestrial, is detected, the division in which it is to automatically, so the player must disarm the minefield sent to the unit manually, which could cause a very great delay of time, mainly if the division in question was programmed for a blitz.

Other Defenses:

Engineers will be able to build "AT holes" for tanks (I know this is not the correct name, thank you in advance if anyone knows), there must also be some X time to construct them, these holes immobilize the armor and they can only be move again when an engineer destroys this obstacle, at a time X, and a part of the armor can not be saved and will be destroyed ...

One defense to delay and commit losses to ships by train is the implementation of barbed wire, which increases the time of landing and this field of barbed wire can only be destroyed by destroyers, cruisers and stranded.

Passive Ability:

In order to realize the concept engineer, they can repair damaged buildings by other countries in the province in which they are located, and do so with less resources than would normally be something like 20% cheaper, say, and there must be an anti-cheating measure where a building can only be repaired when another player if you have attacked that province or if you have recently conquered it.

The damaged building can be repaired automatically when the province has no chance of a rebellion, but when it starts repairing, it will waste its resources, so there should be a new button called "automatic repopulation" or something like that where it is allowed or not the unit will spend its resources to repair damaged buildings, there will be no priority repair option for players without high command, given that the standard repair priority of engineers always targets the most damaged building, but you can stop the action but the Discount bonuses will only target the most damaged building.

They can also build strong, automatically when the period in which this unit is in the province is greater than 1 day, and does so without cost, but the fort can take 1.5 to 2 more time than it should if it were done normally (but this is optional, this can still be discussed).

High Command Options:

For the high command, I thought more about the detecting part and disarmingly, where mine-hunt ships and engineers would have an extra option in which case a naval or land minefield would be found, then those units would have the free will to disarm them or simply go over, so there is no need for frequent attention to this problem

And there will also be the option to set a building repopulation priority, for example, let's say the most damaged building is a level 2 naval base, and you want the building that is first repaired to be the Infrastructure, so you can change the priority of the engineer in the province in question then the action of the engineer changes automatically.

Research and Production:

The unit certainly goes to the technological wing of secrets, and must succeed the command, for example, when you search light tank, you unlock medium tank, and when you search level 2 infantry you unlock motorized infantry and etc ...

So you can research engineers when you pick up commands, engineers unlock on the 16th.

Now the production requirements are barracks level 2, infrastructure level 2 and industrial complex of any level, and because it is a unit of skills so useful, they can only be produced in urban central provinces.

Conclusions:

This unit can give players new tactics and fighting strategies and will be extremely useful, although in a way it can modify the mechanics of the game.

Well I accept suggestions and am grateful now!