Is it legal to make a civ that is functionally similar to a DLC civ to circumvent paying for it? I feel like that might be stealing intellectual property, since without having bought the DLC, Firaxis has not given you a license to use their ideas in other mods. (When you purchase Civ, what kind of licenses does it come with? It's possible you don't get a license to use their ideas in other mods even when you buy things, but at the very least they'll turn a blind eye to it).
You can make an Inca civ, or a Korea civ, and it won't be a problem. But if you create a mod that adds a DLC-civ copy that is totally functionally the same, then you've probably crossed a line somewhere, even if you don't take the art/music assets.
Personally, I wouldn't feel comfortable making a mod to circumvent paying for DLC. I hate the idea of DLC, too. But there's not really anything you can do about it. It may be a "ripoff", or "not worth it", depending on how you see it, but the fact is that everything is worth what the purchaser will pay for it, and the only way to subvert DLC is to make it unprofitable for the companies to release them.