There are two different mod types, here. There are niche/derivative/themed mods (all fit similar groups) and there are non-themed content mods.
The former are easier to come up with ideas for and name--and those who are interested will be interested and usually don't need to be sold. These are mods like my SCP, DBZ, and Fortbent mods. These are different because:
People play the SCP mod because it's an SCP mod. I honestly can't think of any other reason to
. It's kind of shit at this point due to the fact that there aren't actually that many SCPs in, because sciencey magic-like stuff is hard.
The DBZ mod is horribad in fort mode (I really can't think of how to make it better; it makes dwarf play impossible and saiyan play is tantrumy) and great fun in adventure mode, but people do download and play it, probably because, hey, cool, saiyans.
Fortbent isn't downloaded often (617 downloads Fortbent vs. 3707 My Little Fortress) because it fits into a small niche (Homestuck readers), despite the actually-fairly-interesting content (I find new and fascinating things every time I play!). And that's okay. It's a niche mod. I'm not expecting much.
Then there's non-themed content mods. Mods like Genesis, Masterwork, What Has Science Done, Civ Forge, Legend of Forlorn Realms, etc.
I call these non-themed, but that's not quite right. For these mods, the themes or patterns in are usually spread out; they have different themes for different industries and different pieces of content. For example, Masterwork focuses on higher FPS and accessibility, while still adding a ton of content, a lot of it very magic-based (Slademantine, all the various cultists that transform into other creatures, etc.). Legend of Forlorn Realms has a tech-tree theme to it, somewhat, but it also has some very interesting creatures.
The most important part, though, is that the former--niche mods--don't usually need to be sold on content and the latter does. Usually, people
know what they're getting into if they're downloading a mod based on their favorite anime/video game/webcomic/what-have-you, or if the mod is something like Elf's "Kaiju mod" (If you know what a kaiju is, there's about a 100% chance you know what it's going to do to your fort
). For non-niche mods, though, selling content is needed. LFR has its colors, lore, and fanbase. Genesis has Deon. Masterwork has its customization and init settings UI. What Has Science Done has an intimidating flowchart showing you how you extract aluminum from Bauxite. (Which may explain why I only have 127 downloads
)
I'd say people would play a Persona mod. Many people like Persona round here. A kaiju mod sounds fun just because more megabeasts are always fun (unless they're stupid overpowered, but since it's me saying that, that's pretty ironic, isn't it? >_>)