One particularly cool covenant is a "CoinPile".

Basically, you put a set of N coin holders into a single address under a CoinPile.

The CoinPile Covenant is a FE'd up Covenant. The function can accept a list of arbitrary internal-transactions that makes adjustments to the output tree structure and then "rezips" them into a new output. It also makes an op_return output (or e.g. annex) with an encrypted update list encoding (can be compressed w/ key IDs and minimally encoded amounts). As a further improvement, any number of the keys can be re-blinded after each transition, with a provable per user key rederivation per user in the pool (encrypt the key tweaks per user).

The FE covenant "pop" allows a transaction that deletes any number of database rows (with authentication) and puts them into onchain outputs.

This way, an unlimited number of users can non-interactively share an output and the onchain load is basically limited in size to a couple of bits per update ocurring internally, eliding all signatures.)

On chain it looks like single sig.