Package management is a hard problem to solve. You get to answer difficult questions...

• How do I handle dependencies?
  
• How do I ensure compatibility between software?
  
• How should I rollback versions?
  
• How do I keep track of updates?
  

...And even more!

If we use non-distro specific terms, your idea would be a kind of hub where people can submit recipes to build and install any kind of software.
This is a marvelous idea, that [/url=https://crux.nu/portdb/]predates Archlinux[/url]! Amd I said that, not only because I love communism, but because it has many advantages, like early release, huge choice of packages, big efforts shared accross users, you name it.
Unfortunately, just like communism, it also has flaws, that are to be taken in consideration:

• orphanage: people create ports once, and give up on the realeases
  
• duplicates: this is the response to the above issue
  
• mediocrity: not anyone can write clean recipes
  
• chaos: organisation is impossible in this mode
  

I realized that while I was the happy maintainer for a hundred crux ports. Even though I put some effort in keeping my ports up to date, usualy people made their own port when mine where outdated rather than notifying me. Some even recreated them, just so they could put their name in the comments instead of mine!
I also realised that after cloning ports trees from multiple users, I had multiple ports named differently for the same packages, but at different versions, and I had troubles knowing which one was the most up to date.

The main problem for this approach IMO is that contributors must be honest, which is not going to happen soon.
Another barrier, is cross-distro compatibility. You will never be able to provide a build recipe that will work on ARM, x86, Glibc and musl at the same time (I don't think even pkgsrc can't achieve that!).
As for integration with multiple package managers... It's a lost cause already unfortunately. Your best bet would be to use them all at a time.

I still believe this is a good idea, bjt that would only work with a small community, to keep things under control. What do you think?