Are we talking Minecraft mini-game mods & plugins ?
...Or you're talking full fledged stand-alone games ? If so, what is your target platform ? i.e. web-app, native executable, phone-app ?
Personally, I would LOVE a World of Warcraft type game where...
1) you didn't have to level-up
2) Raids are completely random
Since the raids are random, you cannot simply lookup strategy guides... instead you are forced to work as a team and figure it out... Kinda like a team puzzle.
Oh... and minimal trash (if any) before the boss encounter... And if there is a little bit of trash mobs, then it would be to only serve as a hint for the up-coming boss fight.
You will need help designing millions of random raid bosses & puzzles.
So... If you don't level-up, then how do you learn your class ?
After character creation, you go thru an optional class tutorial -- I say optional, because you might already know how to play that class...
Then you take a solo-test in order to test your skills and get rated on how well you play that class.
If you don't like your rating, you can always go back & try again... or use it to practice.
Obviously, the test should gradually increase in difficultly and also be impossible to complete... Only rating & measuring how far you progressed thru the test.
So... it's basically a Team-Puzzle-MMO.
You could probably get a Dungeons & Dragons license for it...
Or come up with your own branding.
The upside is that since you don't have leveling up, then there's NO WORK in creating all those zones, story-lines, & quests.
Once you create the tools, then it makes it easy for the Admins to pump out tons of different raid puzzles.
Also, a lot of people (like myself) got burnt out on WoW and took a temporary or permanent break... But I believe a lot of people might be inclined to come back to a new MMO... especially one without leveling that would be more friendly to casual players.
At peak, World of Warcraft had as many as 12 million subscribers... That's what ? $120 million per month in revenue.
If this idea isn't your cup of tea, lemme know... I can offer other ideas.