Be careful what you wish for ;p :
Scrolling the tech tree of the Caveman 2 Cosmos mod for Civ4 :
https://www.youtube.com/watch?v=eWxAn5gXb2A
(not sure what the exact count is these days, but it's on the order of a thousand techs)
Mod presentation : https://forums.civfanatics.com/threads/caveman-2-cosmos.2885...
Latest major version released last month and Player's Guide : https://forums.civfanatics.com/threads/caveman2cosmos-v43-pl...
Modding is kind of in tension with the concept of direct sales... for similar reasons that open source is in tension with it. It's even worse for games, since most games are closed source, and mods generally directly build up on them.
I could see total conversion mods perhaps separately selling their art assets like this (since the open source of the rest of the code already often is in tension with the level of ownership that artists tend to claim over them).
The closest we got to what you suggest is probably StarCraft 2's Arcade mode, and platforms like these are bad for reasons too obvious to state on HN.
But Civ4 is a great example in how you can create one of the biggest and longest lived modding communities out there : you just need to open up your game ! And this is something that dates to Brian Reynolds' Civ2 : https://www.filfre.net/2023/01/sequels-in-strategy-gaming-pa...
Sadly, Civ5 was a regression there, and looks like Civ6 even more so ?
Speaking of educational content, here too Civ4 seems to be unsurpassed in its Encarta-like Civilopedia, and mods have only improved on the depth and breadth (if maybe not accuracy) by often cribbing their descriptions from encyclopedias :
https://www.youtube.com/watch?v=7WveveonsHM&t=495s
https://www.youtube.com/watch?v=7WveveonsHM&t=1326s
https://youtu.be/-titvSZsurE?t=945
(this player ignores the historical descriptions, but spending minutes reading them is something that most viewers are not likely to be interested in)
Personally, the regression from Civ4 to Civ5 in their Civilopedias is for some reason what pained me the most...