This is a feature that I would really like to see in FotS.
With the setting that this game takes place it only makes sense, and I don't really understand why it was omitted from the game in the first place.
Some might argue that this would throw off balance of the game by making spear units redundant, but I am of a view that it will only add a more interesting dimension to the game. Also, I like to design my army based on authenticity, so I don't enjoy bringing old fashioned spear units when I'm fielding modern rifles (similar issue stands for bringing yari ki, which I prefer not to bring with my modern army - but that is another topic).
I imagine that being able to add a bayonet would be a great feature, e.g.,:
*Make it an upgradable skill (perhaps at level 5 at a cost of around 100~120 koku)
*Maybe make it a feature that you can turn on/off (it would take a couple of seconds for a unit with this skill to fit bayonets on their rifles)
*The benefits of turning this skill on could be like the following:
-Units with this skill on will engage in melee fight with their bayonet-rifles (not swords)
-Adds a small advantage against cavalry in melee combat (e.g., about +5 bonus melee attack against cav units)
-Maybe increased melee attack? (Some might argue otherwise, but there are many historical instances when bayonets triumphed over swords)
*The disadvantages of turning the bayonet skill on could be:
-Slightly decreased rifle aim
-Slightly decreased rifle reload rate
-No bonus against cavs
-Base melee attack with no added melee bonus
I would guess that having this feature would add a whole new style of gameplay in some circumstances, and I would really like to see something like this in the future.