@OP
Of course it would be amazing, but there are countless problems with it. The tl;dr is that it's just too much work, but if you really want an honest answer, here's my 500 word essay response
So realistically, if this were to happen, the best thing to do would be to use some existing video game engine like Unity or Unreal Engine, etc. Not only that, it would almost certainly have to be a 2d game with an isometric perspective with 3d models rendered in 2d, just like Diablo I and II.
2D isometric rendering means I could make really basic assets that would be rendered as low resolution images that could then be edited and imported as sprites into the game. It means I could also make things like houses and building that only require 1 or 2 sides to be detailed. Since the player only sees 1 or 2 sides of a house for example, you'd basically just be making half an asset. Also, rigging and animating everything would be much easier because you could get away with poorly done animations at a low framerate. However, I haven't learned rigging and animating in 3d yet and these aren't particularly easy things to master. They're very complicated and people go to school for years to learn them. I have no education in any of this. I learn everything myself.
Here's an example of something I did recently:
First you make basic 3d models and texture them:
Then render in isometric view as sprites that you can then edit:
At the moment, as far as I know I'm the only dedicated artist on the team (I guess you can consider me "on the team," but I don't do much) that can create these assets and have them look decent. So literally every asset would have to be made by one person. Finding other people who are dedicated to making a game for free probably wouldn't be easy and would probably even make things worse. People working for free can very easily lose interest and stop working without consequence.
Essentially, we have a few programmers and basically 1 dedicated artist. Plus, everyone pretty much has school and/or jobs to deal with so this would be free time (1-3 hours probably every week day, if that) all for no money. We would also have to come up with completely new lore and create an entirely new, unique world with tons of characters and a quest line that we would all agree with and understand. Games need to be planned; you can't just start and expect everything to fall into place. That's why concept art is so important. Also, keep in mind I'm only talking from an art perspective. This doesn't include the amount of work that would go into programming. There would also have to be sound effects, music, maybe voice acting and cgi for cinematics. All of this would have to be outsourced, which means a ton of money spent for just that.
As for crowd funding and getting donations to support the development, while this would be helpful, ultimately it's a very bad idea. No one has made a game of this scope before. It can very, very easily fail. I really don't like the idea of people giving money to help create a game that will take years to create and then potentially not even get released in the end. It would all be for essentially nothing and everyone would feel shitty and upset.
That being said, I do think we could create some kind of game post-sigma. Of course, anything can happen. Maybe sigma will keep developing for years or maybe some new version will be created. If not, I think we could make a smaller game; one that is completely different to the Diablo style RPG. Maybe a horror or adventure game. It's just too early to even have the discussion and I have no idea if I'm alone in wanting this. Maybe no one on the team has any intention in doing anything outside of Diablo.