Make it easy for users to manually create their seed phrase so that they don't have to trust a "black box" and allow for encoding derivation path in seed phrase to simplify recovery

How

Use every eighth word from the wordlist to generate 16 word phrases with 128 bits of entropy (no checksum). The most significant eight bits of each word are used as entropy. The least significant three bits of each word specify the derivation path.

000 Derivation Path Not Specified
001 m/44'/0'/0'
010 m/49'/0'/0'
011 m/84'/0'/0'
100 m/48'/0'/0'/2'
101 m/86'/0'/0'

Up to seven derivation paths can be specified if all words have the same least significant bits. If the least significant bits of each word vary, there are 48 bits that can be used to encode meta-data. As long as meta-data is limited to certain allowable values, this provides a mechanism for error detection, similar to a checksum.

The word length determines how the seed phrase should be interpreted. User only needs to know how many words they have and how many words the wallet supports to check for compatibility with this extension
Uses same wordlist to represent the same entropy as a 12 word phrase (could be a revision to BIP39 instead of a new BIP)
Manual procedure is very simple, each derivation path can use a shortened 256 word list which enjoys improved alphabetical separation of words
May prevent naive word selections which aren't limited to every eighth word (similar to what checksum does)
Can be extended further. For example, a 32 word phrase with the same entropy as a 24 word phrase could also be added. We can keep adding formats with unique word length and keep adding uses for the meta data as needed.