So far I'm not aware of any viable concepts for creating a blockchain-less cryptocurrency that doesn't rely on centralized entities for either token issuance, transaction security or even both. And I'm sure it's not for lack of trying.
- low speed of transaction approval
Before the mempool was congested as it is now 0-conf transactions were viable for most use cases were fast transactions were of essence, allowing for basically instant transactions. With the development of Lightning Network we'll get instant transactions again by being able to use payment channels.
- "change-back" transactions
Trusted counterparties simply send you your coins back if justified, so you can already do that. It's just that you don't have a third party able to enforce such a return transaction for you.
Making transactions reversible by default without both parties consenting or involving a third party as arbiter makes a payment method useless.
- inequality in "proof-of-work" process
Any PoX process requires expenditure of resources in one way or another. Thus there will always be "inequality".
If you are referring to ASIC vs GPU/CPU mining -- there are plenty of alternatives out there.
- unmanageable "input-output" transaction process
Unmanageable how?