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.
I don’t know if it would be a fit for the fediverse, but I do know of what seems to be like a great community that has seemed to survive for a long time where others have long gone by charging something like $5 to sign up one time.