This post is intended to show some of the behind-the-scenes design process. For our example, we'll use a newly proposed feature called Metallised. Metallised weapons and shields are made entirely of metal, rather than having much of their mass in the form of wood.
I first used the Alternate Materials page to determine the cost difference. Converting an item from oak to steel increases the cost by about two and a half times. Unlike the Alternate Materials feature, the density of an all-metal object would be much greater: the item would now be about fifteen times heavier.
There should be some mechanical effect of turning your quarterstaff into a solid iron bar, or making your axe's handle out of steel. One possible solution would be to look at the Dense feature. To replicate the increase in mass, this is similar to taking that feature fifteen times. However, that would also add a +15 to attack rolls, and bump the load of a medium encumbrance weapon up to Heroic+12. The cost would also be much, much higher than the 2 1/2 times we figured earlier.
Forte is another possible feature to model the effects of metallising. This feature gives a +1 to parries and damage, which works well with the solid thwacks against both flesh and steel that a metallised object should have. Forte doubles the cost--not too far off already from what we already figured--and has no inherent weight increase.
Heavy may also work well. The damage value is increased by two, pretty much bumping the damage up to the next higher die. Weight and cost are increased by about a third, and the added mass adds a -2 initiative penalty.
It could be argued that metallised weapons are an example of Iron-shod taken to the extreme. This adds a +1 to parry for an increase in weight and cost of 10%. As the Iron-shod feature presumes that the weapon isn't extraordinarily heavy, and is an entirely defensive feature, I don't think that this one is quite what we are looking for as a model.
Mass: There's a feature that's pretty much a given, no matter what else we do here. This doubles the weight, but doesn't add to the cost. There is a -3 additional initiative penalty, but no bonuses are added. We'll need to keep looking through the extant features for more inspiration. The upgraded version, Weighted, may be closer to what we want. Weight and cost are trebled. The weapon gains +1 to attack and damage, but would also have a -2 to parry and -5 to initiative.
Looking back through the seven different features selected to be a model, a combination of Heavy and Weighted seem to be our best bet. As their names imply, these two features are as similar mechanically as they are semantically. Though our metallised weapon would be able to strike an enemy's weapon much harder, it would be much slower in doing so. I cannot really see giving this new feature a parry bonus. Beyond the weight increase, usage of the weapon wouldn't be all that different. Also, with many metallised weapons a far greater proportion of the weight would be in the handle, so I don't think that we'll fold Mass into this feature the way Weighted does.
The added mass would almost certainly add to the damage dealt. I rather like the idea of increasing the size of the dice used. It's a fairly simple addition, and easy to implement. The slow swinging speed would reasonably limit a further increase in striking force.
Speaking of how slow these weapons would be, a -3 initiative penalty seems about right. As the weight increase guarantees multiple levels of load gained, the penalty would effectively be greater. We don't need to increase the penalty here.
As an added little bonus, we'll give this feature a bonus against Weapon Breaker attacks.
Now we just need to put all of this together. I think that we'll copy the description from earlier to start us off. So, here's our final write-up:
Metallised: Metallised weapons and shields are made entirely of metal, rather than having much of their mass in the form of wood. This feature is only appropriate for items that are not already mostly metal. It could be argued that it represents a different metal, such as uranium. It could, but have fun with your cancer. Metallised weapons are much heavier and harder-hitting. They increase the size of the damage die to the next larger size: 1D8 becomes 1D10, for example. The weight is increased by fifteen times, which is guaranteed to increment the load category. The slower speed that the added mass causes brings with it a -3 initiative penalty. Metallising a weapon increases the cost by 2 1/2 times.