Not a large fee. Just something like the local equivalent of $10 USD.
The deposit sits in escrow during a probationary period while the new account holder has to prove they’re here with good intentions. If the account starts spamming, advertising, trolling, or plugging their slop-coded garbage, they get banned and forfeit the deposit to the instance they signed up with. If they interact in good faith for the “probationary” period without showing signs of being an LLM bot / gaming the system / “karma farming”, the deposit gets refunded in full and they exit the probationary period.
If they don’t interact enough to prove their intentions here a good, the deposit still gets refunded in full but the account gets banned.
This would virtually eliminate drive-by spam and trolls while also funding instances and encouraging positive engagement.
Is it feasible? Probably not. Is this an idea likely to be popular? Nope, otherwise I wouldn’t be posting it here. Do I think this is a good idea? Yep.
If you were going to do this, I think the only way to make it work would be some sort of blockchain based crypto system.
You create your fediverse account and then also some sort of blockchain transaction that consumes some nominal amount of cryptocurrency. Then each of your posts gets a signature which authenticates it against that original transaction.
That said, this is a horrible idea and we shouldn’t do it. It would absolutely solve a lot of spam problems, at the cost of making fediverse participation out of reach for an awful lot of the world’s population. Unless we make it cheap enough for them, in which case it is also cheap enough that those in developed countries can create dozens or hundreds of accounts without concern for cost.