During WW2 the core infantry still used rifles invented before WW1, so generally WW2 infantry didn't differ that much from WW1 infantry. Assault rifles were rare yet.
Depends on how you define 'assault rifle' - the British BREN and the American BAR were both fully-automatic weapons that could be fired while moving and were issued at the squad level: the BAR was adopted in 1918, the BREN in the 1930s.
The major difference between 'WWI' infantry and the infantry of the post-mid-1930s was the addition of all the other weapons to the old rifles. At the beginning of WWI, the average infantry battalion (French, British, German) had 2 - 8 machine-guns total, and no other weapons except the bolt-action magazine rifle. By the beginning of WWII that same battalion had 27 - 36 automatic weapons in the infantry squads (automatic rifles in British and US Armies, light machine-guns in German, Soviet or French), 6 - 12 heavy machine-guns, 12 - 36 50 or 60mm mortars, 6 - 8 75 or 81/82mm mortars, and in the German and Soviet armies, 37 or 45mm antitank guns and 75 or 76mm light guns in the regimental units.
It was this Heavy Weapon firepower that made a huge difference between 1914 and 1939, not the type of rifle carried.
So, if we see Musketmen as those with Firelock musket and Infantry as those with Magazine rifle, we cover most of the firearms history. Also, the transition from one to another appeared during the second half of 19th century and matched switch from colorful uniforms to khaki with helmets. This looks like the biggest gear break since the mass adoption of firearms.
I agree that this would be the best of a really bad historical/gaming bargain, but that doesn't mean I'm ready to settle for it. At the least, we should get the Late Renaissance Pike and shot units (tercios, Swedish brigades, Nassau's battalions) with the matchlocks, the flintlocks that have and are providing so many of the Unique Units (Swedish 'Caroleans', British Redcoats, French Imperial Guards, American Minutemen, just for starters), the Magazine Rifle infantry and the WWII infantry. For one thing, each of these has a distinctively different 'look', from the dull cloth and metal helmets/breastplates of the pike-and-shot matchlock men to the bright coats, tricorns, bicornes, shakoes or kepis of the flintlock period, to the 'semi-dull' period of the smokeless powder ( though khaki is not universal: French were wearing the dark blue coat and red trousers right up to 1914, and the US Army didn't lose its 'Indian-fighting blue' uniform until after 1901: there's still room for color!)
One thing that might solve the problem of Too Many Units, Not Enough Tech Tree - which I can see looming in Civ VI, based on the published pictures of the truncated Tech Tree, is to have 'Promotions' to get new units and unit graphics without requiring specifically new Technologies.
Example: The Matchlock musket requires new Technology: gunpowder. The flintlock requires new Technology: something like Precision Metalwork to manufacture the 'firelock' for each musket.
BUT the conversion to blackpowder rifles requires no technology: rifles were being built in the Renaissance, it's just before the 'Monroe Effect' (look it up!) was discovered, the rifle wasn't practical on the battlefield. That chemical-physics discovery, coupled with Factory production, made it possible to supply rifles and try to use exactly the same tactics (see the American Civil War for how well That turned out!). That could be a simple 'Promotion' which allows you to spend X Gold per unit to convert to black powder rifles, and change the graphic on each unit from shako-wearing red/blue/white/green-coated flintlock fusilier to kepi/pithhelmet/pickelhaube-wearing rifleman.
Then, the magazine smokeless powder rifle requires new Chemical Technology of smokeless powder, but that is an outgrowth of Dynamite and Artificial Fertilizer development, so could be 'folded in' with one of those more general Technologies, and result in the khaki/feldgrau/olive-drab infantry of the twentieth (and late 19th) century.
Which would then be 'Promoted' by a card labeled, say, Stormtroop Tactics or Infantry Firepower which allows you to combine Machine-gun and Magazine Rifle units into Infantry - with a graphic change to the WWII-type uniform.
This would not be inconsistent with the Tech-Civics interaction already in Civ VI: I have only seen parts of the Civics Tree, but I immediately noticed that some units from the Tech Tree in Civ V, like Caravans, are now available through the Civics Tree - let's extend that to the gunpowder and other combat units.
Come to think of it, I haven't seen any part of the Civic Tree for the post Renaissance period - maybe that's exactly what they've already done...