Pack of Familiars Homebrew Character

My players want to create a wierd Homebrew character for one of them to play for a short run mini campaign: A Pack of Familiars. The idea is that all the spellcasters have a familiar that 1 player controls. the Pack can do the scouting and helping and touch spell casting and what not just like a Familiar would do, but also have the pack, under 1 player's control, work together to come up with creative solutions to unique problems that a single Familiar couldn't do.

Sounds fun in a Chaos Goblin sort of way, which is our normal mode of play. What we're stuck on is how to turn this idea into a playable character with level advancement and the normal things that come with a character.

The ideas we have for advancement (in no particular order) are:

The Pack will gain otonomy, choosing their animal and with spell slots and a spell list of touch only spells. 

The options of animals gets expanded beyond CR 0 eventually becoming the equivalent of a Ranger's companion or a Paladin's steed

The Pack gets to take the Attack action (1 Familiar attack per round. Not an attack from each familiar)

The Pack gains telepathy and can talk to each other and the rest of the party.

After that, we're stumped. Anybody have other ideas for advancing a pack of familiars character and/or in what order would you put these level ups?