The spam is the biggest issue. We never solved it there, and it's only being controlled on usenet's imitators through tight control, which is not a feature of usenet.
One setup I could see working is a pubkey setup where anyone who posts, always encrypts with their private key, and anyone you want to hear from, you add their public key to your list of keys you decrypt to read.
Aggregators can occupy public keys as their "address" and any aggregator that gets obnoxious, you just trim it from your list, which only contains things you've either let in or accepted via slates or whitelists.
Then the spammers can do whatever they like, really.
I believe the main reason usenet faded so thoroughly was that it became a piracy distribution platform very early on (not sure which came first, the pirate booty or the porn booty, but I'm guessing porn), and very quickly there came a kind of unspoken "don't talk about usenet" code among those who already knew about it. At the time, Napster and bittorrent were the main targets of the authorities and IP trolls and usenet was just doing a Jim-From-The-Office-smirking-through-the-blinds.