Zerocoin is an example of a true innovation as I see it. Zerocoin does not rely on mixing as Monero does, but implements true removal of all history. This is something that neither Bitcoin nor Monero can do. If someone made this, I'd seriously consider it.
http://zerocash-project.orghttps://github.com/scipr-lab/libsnarkhttp://www.scipr-lab.org/How are Zerocash transactions checked for correctness?
For a mint transaction, the commitment cointained therein is constructed so that that anyone can verify that the committed coin has the claimed value.
For a pour transaction, anyone can verify that the zero-knowledge proof contained therein is valid (and that a few other simple invariants hold). For efficiency, however, Zerocash does not use "any" zero-knowledge proof, but leverages zero-knowledge Succinct Non-interactive ARguments of Knowledge (zk-SNARK) systems, which are zero-knowledge proofs that are particularly short and easy to verify. Specifically, Zerocash uses zk-SNARKs constructed by SCIPR Lab described in BCTV13; such proofs are less than 300 bytes long and can be verified in only a few milliseconds.
If I recall correctly, the Bitcoin dev team was really excited about Zerocoin, until they saw that computational overhead which rightfully scared them away.
Now a zerocoin sidechain that is merged mined with Bitcoin and only lives off of it's own transaction fees, would enable people to fully clean any coin they have for a modest fee.