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?

I don't think you should make a class for this. I think you should create a race and then the character could be a wizard or other spellcaster. This way, it would be much more balanced and easier to design.

Is the idea that one of the player's doesn't have a character, but instead plays the game as the familiars of all the other player characters? I'm going to assume this is the point so that the "Pack of Familiars" is the entirety of this player's character, rather than a companion animal like a paladin's steed or a ranger's companion. Otherwise you'd be asking about a subclass for another class where this player has a full character of some class and then their subclass enables them to use the other characters' familiars as somekind of companion, right???

Class: Pack of Familiars

Hit Dice: 2d4 per level
Primary Ability: Dexterity
Saving Throw Proficiencies: Dexterity, Intelligence
Skill Proficiencies: Choose three from Arcana, Deception, Persuasion, Perception, Nature, Survival 
Armour Training: None (you are physically unable to wear armour)
Weapon Training: None (you are physically unable to wield weapons)

Pack of Familiars Class Table:

Level 1: Wild Strike

As a pack of tiny animals, you can make strikes using your various natural weapons. You can use Strength or Dexterity for attack and damage rolls with your Unarmed Strikes which deal 1d6 piercing damage on a hit, when you take the Attack action you can make one unarmed strike as a bonus action.

Level 1: Spellcasting

You use your Intelligence as your spellcasting ability modifier for your spell attack rolls and your spell DC. You learn 2 cantrips from the Pack of Familiars spell list, and you prepare a number of 1st level spells as shown in the Pack of Familiars Class Table. When you gain a level in this class you can change one of your prepared spells to another on your list.

Level 2: Cooperative Forms

All the constituent creatures making up your pack can work together to form themselves into new useful form. Choose two forms to learn from the list below, you learn an additional form at levels 5, 9, 13, 17. When you gain a level in this class you can swap one known form for another. 

Changing Forms You can use a Bonus Action to cause your pack to assemble itself into one of your known forms. You can stay in this form for up to 1 hour before changing back, you can also change back as a Bonus Action. You can change form a number of times as shown in the Form Changes column of the Pack of Familiars Class Table, you regain all uses when you finish a long rest, you regain 1 use when you finish a short rest.

Forms:

Humanoid Form: Your various constituent creatures can organize themselves into a fascimile of a humanoid creature. While in this form you gain proficiency in Light armour and simple and martial weapons and can use them as a humanoid creature can. In addition, you have advantage on Persuasion and Deception checks. 

Vehicle Form: Your constituent creatures organize themselves such that they can lift up an carry one Medium sized creature, this increases to two Medium creatures at level 10 and 4 Medium creatures at level 15. In addition, your movement speeds increase by 20 ft while in this form.

Container Form: Your constituent creatures organize themselves to carry items in a disguised manner. Your carrying capacity is doubled, and observers cannot see what items you are holding while in this form, this increases to triple your carrying capacity at level 10.

Serpentine Form: Your constituent creatures make a chain, enabling them to act like a rope to aid others to climb, swim or otherwise cross hazardous terrain. You and friendly creatures adjacent to you have advantage on Athletics Checks and Dexterity saving throws. 

I like where you're going. I not sure I vibe with the Voltron forms bit. I'll have to think about that. I wasn't considering hit dice or HP progression. I figured they get the HP of their animals and die just like a familiar would, only to be recast the next morning by the other players. We have 1 player who is extremely creative so  the other players want to emphasize the chaos of having that player play whatever animal is chosen for them. 

I was thinking that they start out as true familiars with a progression more like the Druid's wild shape where they get to be slightly stronger creatures as they progress, eventually being as capible as the Ranger Companion or Paladin steed but not directly tied to the characters by that level. They'd be attached in the early levels and gain their independence So they wouldn't be able to attack or get their own spells till like level 3. And the teamwork would be more direct and true. Like, "The owl picks up the weasel and fly it over the wall while the cat distracts the guards."

The more I think about it, the more it's like a really weak Druid who can't do much but can wild shape all the time and there's 3 of them working together. Strong in exploration and infiltration, weak in combat and social interactions. (This is an intentionally unbalanced character because the player's skill and experience is so unbalanced in the opposite direction and wants a challenge). 

Oh! I like that approach. And the player can just roll play the player/familiar interaction. And I think a Cleric would be a better class fit to emphasize the support/healing that is normally associated with familiars.

Familiars IME aren't really associated with support / healing like a Cleric, but rather the exploration / utility of a druid. If the point is for the character to be not equivalent of a player character, then I'd suggest using the "Expert" Sidekick rules from Tasha's as the basic framework which gets added on top of the baseline abilities of the animals they currently have access to. Pick a hit die size for them, probably a d6, which determines their hit points. They still only get one Action, Bonus Action, and Reaction, but they can swap between the senses, special abilities, and skill proficiencies of each of their constituent animals. 

At level 5 they gain the ability: 

Independence: you can split into three Tiny creatures thus occupying three different positions at once, when you take the Attack action you can attack from any of those locations. In addition when an ally casts a spell, you can use your reaction to have that spell originate from any of the locations you currently occupy.

Otherwise they get the abilities found in the Tasha's Expert Sidekick class. 

Alternatively, if the other players want to sort of dynamically nerf this experienced player. I'd suggest the other players create three different Sidekicks, one of each Sidekick class to represent the different familiars. Each Sidekick class would get attached to one familiar animals type - e.g. they could make an Expert Owl, a Fighter Snake, and a Spellcaster Frog. The other players then get to pick - by casting Find Familiar - which one of those three forms the pro-player is playing. This also gives you some options to allow the other players to change their minds by making additional SideKick characters to choose from later in the game.