That would be cool! I have very little experience with modding art.
I believe there's information on how to do so in the modding guide on the Civ5 - Modding Tutorials & Reference forum. There might also be more tutorials there on how to mod artwork. I've attached the most recent version of the ModBuddy project if you'd like to take a look in it.
@Sneaks
I really like the idea of a friendly-adjacent bonus! Sorta like the Discipline policy? I don't see a friendly-lands movement bonus option, but there's one for friendly-lands combat (Himeji Castle's effect).
I have found myself not creating any more swordsman, now that I have the choice, when in doubt I always go for militia. ¿Am I missing something? The swordsman is a bit stronger, but it has the requirement and costs more, I guess the promotions are different, but their role is similar.
The difference is combat bonuses. If we consider an average early game war...
20% Great General
20% Discipline
20% Terrain promotion (swordsman only)
20% Flanking bonus from at least one unit assisting
12 * (1 + 0.2 + 0.2 + 0.2 + 0.2) = 22 Swordsman (on attack and defense)
10 * (1 + 0.2 + 0.2 + 0.2) = 16 Militia (18 on defense)
Strength is non-linear so the +40% strength of a Swordsman has more than a 40% effect. Swordsmen can also get the +35% Siege vs cities as their second promotion, while Militia only have defensive bonuses. Militia are also at a later tech level than swords.
Since these are mostly advantages on the attack, swords are good for a conqueror but not needed for peaceful games.
I think the problem with the "swords can also attack" logic is that I (and I would imagine quite a few others) primarily use ranged attacks to take cities and kill units. The ground troops are entirely there as a buffer, to protect the ranged units. Therefore the defensive bonus on the militia make them overpowered for this role, even from an aggressive perspective.
The main thing about comparing swords vs militia is militia come too late to be effective in early warfare. I've usually conquered several cities before reaching the Medieval era, and my classical priorities are ones good for conquerors (iron working and construction) not peaceful players (philosophy).
Still, since the number of strategic units is limited I'd be okay with buffing their numbers further. Perhaps 14 Swords and Horses?
The main thing about comparing swords vs militia is militia come too late to be effective in early warfare. I've usually conquered several cities before reaching the Medieval era, and my classical priorities are ones good for conquerors (iron working and construction) not peaceful players (philosophy).
Still, since the number of strategic units is limited I'd be okay with buffing their numbers further. Perhaps 14 Swords and Horses?