Thanks for the proposal. Aside from potential privacy implications of revealing the derivation path for non BIP-44, 45, 49 HD nodes, this scheme is superior to the alternate {x,y,z}pub idea. Since coin_type is part of the path, the 'xpub' prefix may be shared across all coins if they so choose. This format provides lock-in for BIP-49 going forward and opens the door for a parallel native-SegWit BIP.
The birthday field is interesting. Could you provide some motivation for its inclusion?
Could you also add some test vectors?
There are a few minor grammar / spelling errors, but we can nitpick those after this goes to the pull request stage on bitcoin/bips.