Let me explain something to you called inputs and outputs. In a transaction, you will use the inputs to pay the outputs. Simple. The more inputs there are, the larger the transaction size. The more outputs there are, the larger the transaction size. Typically there will be two outputs because there needs to be a change address. Now, what happens if you have a hot wallet that uses 4+ inputs? That's a BIG transaction.

The point I'm getting at here is that even if you are able to declare the fee, you run into the problem of your declared fee being useless because of the size of the transaction. You could post a 0.005 BTC fee yet still have it confirmed slower than a transaction with a 0.0008 BTC fee simply because the fee:size ratio is smaller. This will still lead to complaints. And if you declare a system where users pay in sat/byte then you have wholly new problems when they are paying huge fees because the transaction takes 10+ inputs.

As I have written previously, one of my withdrawals from another site [2f1459fcc40cb6af51a5382a1f4847a1bbf7d8390dd4ffd3ef82b0ab1379ec2d] used a 2.8 mBTC fee which would still take a long time because the effective rate was 150 sat/byte.