Maybe this has already been discussed, but I have not found anything online.

To the best of my knowledge, the only BIP which specifies a HD structure
for multisig wallets is BIP45. Unfortunately, when used in a
multi-account fashion, BIP45 gets very tricky very fast. In fact, one
has to either use a new master for every multisig account (hence having
to backup many master private keys) or use the same master for many
multisig accounts, resulting in deterministic but complex and
undesirable key reuse.
I would like to propose a new structure for multi-account multisig
wallets. This structure follows the derivation scheme of other proposals
(in particular BIP44 and BIP49) but adds a level to take into account
multisig accounts separation. In particular, the structure should be as
follows:

m/purpose'/coin_type'/account'/cosigner_index/change/address_index

In this case, a user can create many multisig accounts (each one will be
a different account number) and give his/her account's public derivation
to the cosigners. From this point on, the creation of a multisig P2SH
address will follow the same procedure as described in BIP45, with each
cosigner selecting his branch from the other cosigners' trees.

Would this proposal be acceptable as a BIP?

Simone Bronzini