Tanks are armoured vehicles used by many of the world's militaries for frontline combat. This blog post outlines key points of a tank's design, and suggestions of how to incorporate these factors into HotM stats.
Tanks are usually controlled by four people; the commander, the driver, the gunner, and the loader. The tanks effectiveness will be drastically reduced each time a crew member is killed or incapacitated. However, the tank's armour will likely protect them from most weaponry.
The key advantage of a tank is its armour, which allows the tank to operate under fire while still carrying out its objective. Most modern tanks, such as the M1 Abrams, use "reactive armour", which uses a minute explosive to adjust itself and thus recover from most hits. The downside is that reactive armour can only react once, as the explosive cannot be replaced on the spot.
Suggestion: a plate of reactive armour will afford the tank with 75% damage reduction the first time it is hit. Afterwords, the rate of reduction is brought down to 40%. Tanks equipped with more standard armour will be afforded somewhere between 40 and 60% damage reduction, depending on the model. Some weapons are specifically designed for piercing tank armour.
The primary weapon of most tanks is a high-caliber "tank gun", which may fire one of many different types of ammunition. Fortunately we already have stats for some of these, found on many different pages of the wiki but originally on PV: Weapons. In essence though, a tank will carry one large gun with maybe a couple of machine guns as well.
Tanks, due to their treads, are more versatile than many other types of vehicles. However, their sheer mass makes it difficult for them to traverse some areas. On a flat expanse, most modern tanks can reach speeds of 20-35 miles per hour, roughly around the speed of the horses we already have on the wiki.
OK, that was shorter than I thought it would be. Also, I only had one stat-suggestion. Even so, please tell me what you think in the comments. If I get a go, I'll proceed and make an article for it. (What namespace would I use? :O)