What many people do is sell BTC for LTC, then later use LTC to buy back BTC.
There are other options as well.
It needs to be able to be resistant to government-grade analysis, big data and stuff like that. But I agree that zerocoin is not a good idea.
Then we should make a "Zerocoin alt" just for mixing. Set up the service on Tor.
Problem solved?
I think there are other proposals which are better, after all, if you leave the Bitcoin blockchain, you lose the confidence that is brought by the decentralization and infrastructure of the network.(If I were to "launder" my BTCs, I would not use any alt-chain, other than possibly LTC)