Gameplay should always beat historical accuracy. Civilization V was horrible then it came to military unit placement in the tech tree and many units was horrible designed as well.
If you think Gameplay and Historical Accuracy are always at odds, then you don't know enough about the history.
In virtually every case, if you delve deep enough into the history, there's a historical solution that will solve or alleviate your Gameplay problem. Agree or disagree, but that's my opinion based on 50 years of playing games of all kinds and 30 years of writing military history and helping to write game rules and design games.
And in answer to an earlier post, my statistics on range and rate of fire (which I should have modified by saying 'effective rate of fire') are from having personally fired the muzzle-loading black powder rifle, breechloading rifle, breechloading carbine, magazine rifle, muzzle-loading smoothbore, and matchlock musket - to be completely truthful, the last only with a powder charge, they wouldn't let us load an actual ball and risk hitting someone!
You might be able to physically fire 10 - 15 rounds a minute with a breechloader, but you can't see your target until the black powder smoke dissipates, so except for filling the air with random lead (and firing off all your ammunition) you will have to wait between shots to 'acquire' even a target as big as a close-rank line of infantry. Same with the range: the French Chassepot rifle could fire to 800 - 1000 yards, but once everyone starts manufacturing gun smoke all around you, you're firing blind, and it becomes essentially a random rain of lead.
Now, to the specific problem, here's a possible solution:
Change the Matchlock 'Musketman' to a Support Unit.
Here's the argument: ALL musket-firers before the flintlock and socket bayonet were used in support of pikemen, because the musket men had two failings: they could not fire fast enough to defend against a cavalry charge, and they could not use the matchlock musket as a melee weapon. The musket weighed up to 20 pounds, and made a really clumsy club. Drop it to draw your sword, (which were, generally, really cheap 'hanger' short swords, more like a cheap machete) and you are no longer a musket man, just an unarmored, untrained swordsman.
... So, let's make the Musketman a Support unit, which can be stacked with a pikeman (as I understand it, the ONLY infantry melee unit in the Civ VI Medieval Era) and you automatically recreate in the game the 'standard' Pike and Shot Renaissance Melee formation.
Then, 150 years later in Game Time, you introduce the Fusilier with flintlock and bayonet, and get a Melee infantry unit that can do what the Pike and Musketman did together, all by its lonesome. This also coincides with the introduction of the colorful uniforms of the 18th century: the British 'Redcoat', the Bavarian and Prussian blue-coats, the Austrian and Royal French white, so it makes sense as a Graphic change as well. Finally, it puts the current Civ VI Unique Units for France and England, the Redcoat and Imperial Guard/Grenadier, in their proper place as they were both primarily flintlock-using units.
Then, a little less than 200 historical years, X Game Turns later, bring on the Magazine Rifleman, using a technology of Industrial Chemistry or Dynamite, because the change from Black Powder to Smokeless Powder, I contend, was much more important than the change from black powder smoothbore to black powder rifle, because of the visibility limitations of the black powder I described above.
You could have a Promotion available in the mid-Industrial Era (about 1860 in historical date) to allow Fusiliers to get extra Combat Power representing the 'transitional' black powder rifles which, after all, were only in general use for about 25 years historically.
That would give us the historical characteristics of the major distinctive 'gunpowder' infantry units for approximately 400 years, require only 3 distinct units, and also allow the game to represent the tactical interaction between the gunpowder units and other units in the game: The requirement for pike and matchlock to work together against cavalry, the ability of fusiliers to defend against cavalry so that cavalry-fusiliers-cannon have to combine their effects (in game terms, hit the enemy fusiliers with cannon, Then charge with the cavalry - the factors should be such that a headlong cavalry charge against intact Fusiliers will usually fail, as they did historically), and then the advent of 'universal' infantry with the Magazine Rifleman in the late 19th century.
-And, about 50 years later, the amalgamation of machine-gun and Rifle units into the Infantry of WWII: the fourth 'gunpowder' unit but also one that carries over into the Modern and beginning of the Information Eras.
Sorry to be so wordy, especially since Firaxis may have already addressed and solved all these problems!
