Breastplates and helmets and greaves have been around since ancient ancient times, like bronze-age? They fell out of use in the middle european ages due to the effectiveness of mail armor over padded clothing, though enclosed helmets never really fell out of use, they just evolved.
Pikes, great axes, mauls, whips, flails, those have all been around since ancient times as well.
I understand the focus of this mod is on a particular time period of a specific region of european warfare history. During that time period, certain types of weaponry and armor were uncommon, due to the changing landscape of warfare in the region. Yes, breastplates were very uncommon in 12th century central Europe, because a Hauberk was cheaper and offered excellent protection to the common weapons of the day. That is, until metallurgy and crafting expertise improved significantly, as well as the type of dangers encountered on the battlefield also changed, then breastplates (for cavalry mostly) became the obvious superior protective armor in the late 14th century. Helmets were always a thing, caps were just the most common for your men-at-arms due to cost and weight.
There are significant economic and technological factors relating to the popularity and general use of arms and armor in historical context. The skilled labor and quality metals needed to craft closed helms and breastplate/cuirasses for normal men-at-arms was just not available in early middle ages, so mail and padded jerkins were the go-to solution. Against a soldier in mail, one does not need the armor penetrating power of a war hammer or morningstar, where a properly tapered sword of the oakeshott type XI will penetrate just as well, with greater agility and more combat utility, and where a spear or pike (and they definitely still used pikes) were extremely effective at penetrating mail.
However, before mail was a commonly known armoring art, breastplates, greaves, and all of the multitude of Roman, Mesopotamian, and Persian/Arabic composite and layered armor types were also known.
What I am getting to is this. Breastplates were still a thing, people knew how to make them. Same with helmets and greaves. They were just less cost effective and reliable than mail (at the time). Pikes were definitely still around, and were still a core consideration in warfare.
And I know you are aiming to make an early medieval arms/armor mod, but I don't want to live in a world where dwarves don't have war hammers. That is just insane!