A decentralised "bank" sidechain for btc. Is this an original Idea?

Please note that I am a mathematician and economic analyst, I do not currently write code and have a very basic understanding of cryptography though it is immensely interesting to me! When reading about mimblewimble I had a flash of insight and I figured I'd write it down just in case no one has thought of it yet!!

My idea is to run mimblewhimble side by side with bitcoin as a debit system, a decentralised bank(without fractional reserve lending) whose account holders can transact without fees amongst eachother, only paying tx fees when moving onto or off of the platform. While this would be a separate currency, it's value would be pegged to btc. The "vault" wallet on the btc blockchain would be generated with some form of public randomization and it's private key would never be seen by human eyes. The protocol would automatically log deposits and send withdrawals from the btc "vault" while creating and destroying its own coins at a 1:1 ratio.

This coin would not be mined and would therefore need to be very "light". My understanding of cryptography is limited so forgive me if I misunderstand mimblewimble. My assumption is that it would allow secure peer to peer transactions without the global confirmation aspect, using minimal processing power and running off only users' wallets. Is this a correct assessment of it's capabilities?

Alternately if mining is necessary, a 10%/20% surcharge(of existing tx fee) would be added when depositing/withdrawing btc after the first year in order to generate revenue for nodes and coin holders as well as encourage users to keep their btc in the vualt. The fee revenue allocation would be split between nodes and account holders. It would work something like: (account volume/total platform volume) * (holding duration on the platform/1year), perhaps doubling at 5 years and again at 10. Surcharge would be waived for all participants who sign up within the first six months to catalyze mass adoption.
So with one coin we could alleviate the stress on btc, allow it to function as a currency without the "tax" of tx fees, and provide a truly secure, interest bearing "debit" wallet!

Has anyone had this idea yet? please point out any errors I have made or problems I have overlooked!