1) Make a better client that's bug for bug compatible. 2) Get everyone to adopt it. Easy. bitcoin-core is a reference implementation. Just because 'the code

1) Make a better client that's bug for bug compatible. 2) Get everyone to adopt it. Easy. bitcoin-core is a reference implementation. Just because 'the code base is shit' (it isn't, compared to many I've seen/worked on) doesn't mean it can't be a reference. If you can make a better bitcoind then use it, if it's so much better and you trust it with your bitcoin.