Musketmen aren't good. They would be fine, except that Crossbows are
so much better. I'll explain.
In vanilla, X-Bows last the whole of the renaissance. During that time, they're still capable of taking down cities, although you might also want to use cannons to support. However, you're X-Bows are going to be higher level, because they'll be upgraded from the Comp Bows you used in the Classical era. Therefore, you will be happy to keep with your X-Bows. When Gat Guns come along, suddenly your X-Bows are upgraded into something that's not as strong. Sure, you can make sure that your X-Bows get Range, but by this point I don't think it matters. This is because of 3 reasons:
- The Arsenal has increased city health
- Riflemen
- Artillery are amazing
The first reason's pretty minor - sure, Arsenals increase city health, but that's what the Cannons you've got lying about are for.
The second reason's more substantial - Riflemen have 34 strength, while X-Bows have 13. This means that Riflemen will easily take out ~2/3 of a X-Bow's health. Sure, you can use your Riflemen to protect them, but X-Bows' 18 Ranged Strength doesn't do much against 34, especially if they've got cover.
The final reason is the killer. Artillery is one of the biggest gamechangers. Suddenly, that hilly, mountain fortress you had is nothing. Artillery can shoot from outside the city's range, and over anything. This means that it really doesn't matter if you've got 2-range Gat Guns, or even 3-range X-Bows, because Artillery obsolete both.
The issue is that Musketmen don't have any of these things going for them. Okay, so maybe they've got point 2; 26 vs 13 might do some damage. But the ranged strength increase is so minor that it doesn't make much of a difference whether you're attacking a Tercio with 22 or 18. The difference is that you can hit the Tercio with a lot more 18s than you can 22s, because of the X-Bow's range.
Regarding point 1, adding a new defensive building could help obsolete the X-Bow, forcing you to use Cannons. This could work well, but it would mean a bit of messing around with EE compatiability. Still, could work.
Point 3's the kicker, though. The reason that people are fine with reducing their age-old range units to 1 range is because of Artillery. In RER, there's no equivalent for this - You're just making all your age-old range units weaker. Therefore, Musketmen either need a bonus, or there needs to be something to obsolete them both. Option 1 would be easiest, but I can't think of anything nice. Possibly make X-Bows cost Iron? Or is that too much stuff costing Iron? And it doesn't make much sense, really, that you're going to make a crossbow need an Iron resource when pikes, gun barrels, and chainmail armour don't need it. Or maybe giving Muskets a small bonus against cities, or against other units? Could Muskets get a bonus against Longswords and 2-handers? Could work, but then you'd need to give the same to the Tercio, which would negate the 2-Hander's bonus.
Just some ideas
(Oh, and am I the only one who thinks that Gripsholm Castle is a bit weak?)