I would suggest a few guidelines to avoid being sued:
1. Don't put a price on your mods. Instead, allow interested fans to volunteer to pay you to do your work better. Here's what I mean:
Don't:
-- Charge people $2 for your mod.
-- Suggest that people pay a donation before using your mod.
-- Make people pay an indeterminate sum for your mod.
-- Go mercenary, refusing to produce a mod or update a mod without payout.
Do if you want to go that route:
-- Allow people to donate to you just because they like you.
-- Promise to use their donations to make your mods better, by purchasing better art tools, or hiring artists to work on your mods, or by ensuring that you actually purchase the next Expansion Pack so that you will actually build mods that are compatible.
-- Sparingly (oh so sparingly) allow people to patronize you. I mean, to give you commissions. I'd recommend only doing these one at a time, and being very very specific about the amount of time you're putting into this.
Judging by the OP, I'd say you're leaning more towards the DO side than the Don't side, but it's worth saying something.
Firaxis is only likely to get on your case they decide that you are making money from their hard work. For example, if you're selling a Civilization at 2$ a download, they may be like "Hey! We made the XML actually DO what you're making it do and we're fairly protective of that, you know!" and sue you.