I am amazed nobody has mentioned gunpowder yet when greekfire has already been mentioned it exist well before 1400 although it only became really good during the 1300s and 1400s with the invention of corning the powder which makes it easier to transport and store and if done right increases the power by around three times. Although during this time period except for certain places in the late 1400s where limbered with trunnions field guns and the arqubus where used it was limited to bombards, bombs, rockets, grenades, handguns, and the breach loading swivel gun.
Pretty sure Toady has made statements saying he doesn't plan to have and doesn't want to include gunpowder. Even if its barely in the "1400 cut off" he doesn't think it fits thematically. Anyone got the searching skills to find an exact quote? There are several mods that add it, but it probably unlikely to be added in the main game. That said, if better support for multiple materials in a single item (gun made of metal barrel and wooden stock, bullets with powder and shot) and for projectile weapons (separating reload and fire actions) is added then modders can handle the rest for those that want gunpowder.
First off Gunpowder is not barely within the 1400-1450 cut off by any means. Gunpowder had it's roots in the 9-11th Century AD in China, and was in Europe by the 1200's. Gunpowder saw it's first use in siege weapons in Europe in the year 1262, and hand-cannons made an appearance in the first half of the 1300's. Cannons were significant by 1400, and undoubtedly so by 1422, with cannons being used by both sides during a siege of Constantinople in that year in large numbers, followed by the fall of Constantinople in 1453, in which the Ottoman's cannons were powerful enough and of great enough numbers that the walls fell for the first time in it's history.
As for Greek fire, we know enough about it, while we don't know it's true mixture or how it was made, we know that it stuck to objects and continued to burn underwater. So... yeah that kind of narrows it down somewhat to what it could be based on what materials and technology they had available.
Also, I do believe Toady's statement was we wouldn't see more than basic usage of gunpowder. I believe he said traps are likely, but he wasn't sure of including cannons of any sort.
That said, I personally believe basic cannons, alongside hand cannons and trap usage all have a place in the game, which is why I do include hand-cannons in my mod.
Composite/laminated materials: composite bows, studded leather armor, lacque/leather/metal armor (like samuri), quilted materials, chinese paper armor, etc. For it to work really well (as opposed to mods now that use two materials in a reaction to make a third material), the game should support some way of layering/combining materials and working out properties from the properties of the original. Maybe take the best trait of each material, so for example leather+metal gets leathers flexibility and the metals strength while reducing overall weight?
Medical materials in general, salves/poultice and the like, medicines (as appropriate for time period). The could be made using the existing system for reactions and then applied when appropriate injury comes up. So basically its just an extra medical job in terms of new underlying game mechanics. The raws would give the medical usage and a rating in effiacy. tags like [MEDICAL_USE:TOPICAL] or [MEDICAL:USE:ORAL] for how its applied, and tags [MEDICAL_USE:HEAL_CUT:1] [MEDICAL_USE:INFECTION:3] for when to use it. For extra detail though, have some medicines only useful to counteract certain syndromes, and soem medicines be nothing more than placebos (which still give a happy thought for "received medicine".
Tempering/quenching and other processes made explicit. Like I think right now steel's properties assume tempering, but it could be added as an extra step that alters the metal. Either the metal's stats could give a range of properties based on different treatments, or it could be a fixed modifier across all materials for a given treatment. Boiling could apply to leather for extra strength, heat treating for wood, etc. The material gains a modifier, instead of just the finished item. The realism/simulation and min-maxing players would like it, though the extra detail would annoy other players probably.
Also, just noticed mod in your sig with your alloys. I'll check it out sometime.
1. I too want to see composites in the game, but that I am guessing is planned and there is probably a lot of coding which would be required in order to add them.
2. And as much as I'd like to see medication... again, something which would require a lot of coding. Alum powder and elemental sulfur, and all those other medications of the time would be nice.
3. I'm not sure the steel assumes tempering as the in-game version doesn't fall far from untreated 1060 carbon steel. (as pointed out by my OP in the spoiler box) But given material properties change - that should be shown. Yet at the same time there should be a quality modifier and a risk involved. After all, poorly heat treated metals tend to be weaker than metal that hasn't been heat treated at all. Yet again, something which would require a lot of coding, though at the same time perhaps we could come up with an in-term solution of a simple reaction to turn untreated metal to a heat treated version.