Map packs don't require as many resources to create. You can use its sales to judge the general interest in DLC.
No, you can't really. Users can create their own maps just as easy. There is no market because it is not a limited resource. As a strategy gamer, I understand that content has value relative to rarity. For example, putting some Metal out on the market does not even come close to testing the waters for Elementium. One is available everywhere and one is impossible to get.
Stamps would sell. They are harder than heck to create. Wildlands would sell. I currently have to use the XML, not the map maker, to edit them. New art assets would sell. Maps are maybe worth a dollar for 100. Stamps would be about the same. And this is only even marketable to regular users who would notice a difference.
The best strategy for DLC is going to be giving us things we can't make ourselves. You can't compete with free content. But anything you can give us that we can't get from the modding community is worth alot. Hell, making new tools and polishing the old ones would be worth something as well. If people are going to give you 5 dollars for DLC they don't use simply to support the company, make that 5 bucks for modding tools and ask everyone to chip in. I'll make it worth their while.