My only disappointment about the barbarian is that he can't multi-class as a barbarian with another totem to have more 18-second-metamorphosis options. So here is the shoggoth totem, to create the most PF2 barbarian of all PF2 barbarians!
Anathema
A Shoggoth barbarian must change her form at least once per round while raging.
Bestial metamorphosis rage (totem ability)
The Barbarian get the Bestial Rage power from the Animal totem, with the following exceptions. She doesn't have to chose an animal at level 1; instead, each times she activate her rage, she can chose an from the animal totem list and gain its natural attack. If she does, she lose his ability to use manufactured weapons like swords. This count as a change of form for the purpose of anathema.
She can decide not to activate this power - she doesn't gain the natural attack, she doesn't lose the ability to use manufactured weapon, and she didn't change her form for the purpose of the anathema.
She also gain the following abilities during a rage:
[Free action] Change appendices
[rage, blah, blah]
Trigger: you start your turn, or you end your turn, or you start an action, or you finish an action, or whatever.
Frequency: 1/round.
You change your animal form. Choose a natural attack you have activated, and replace it by a another natural attack from the animal totem list; alternatively, you can chose to gain no natural attack and regain the ability to use manufactured weapon instead. This count as a change of form for the purpose of anathema.
[Action] Grow appendices
[rage, blah, blah]
you grow a new arm with claws, or a new head with horns, or a new jaw on your torso... Choose a natural weapon from the animal totem list: you have access to this natural attack. This doesn't prevent you to use the natural attacks you already had, or a manufactured weapons if you were able to use it. Alternatively, you can grow an arm able to use manufactured weapons. This count as a change of form for the purpose of anathema.
Resistance
When you activate the rage and at the beginning of each turn while raging, chose a totem and make a DC 10 flat check. Success: you gain the resistances from that totem until the beginning of your next turn; if it isn't the same resistance as the previous round, this count as a change of form for the purpose of anathema. Failure: the DM chose another totem, and you gain the resistances from that totem until the beginning of your next turn; this never count as a change of form for the purpose of anathema.
Feats
Special: a shoggoth barbarian can select any feat from any other totem that allow her to change her form. This include, but is not limited to, animal skin, giant's stature, animal rage, dragon totem wings, titan's stature, giant's lunge and dragon transformation. You don't have to respect the prerequisites to select those feats. Obviously, when any of those feats activates, this count as a change of form for the purpose of anathema.
Level 1 feat: persistent form
When your rage ends, you don't automatically lose all the forms you got during the rage. When the rage ends and at the start of each subsequent non raging round, roll a DC 10 flat check for every rage feat and natural attack that's still active. In case of success, it doesn't deactivate until the next roll.
Level 4 feat: [action] double... strike?
[rage, blah, blah]
Chose two attack; at least one of the two must be a natural attack. Make a Strike with each attack.
Level 10 feat: [action] double form
[rage, blah, blah]
Frequency: 1/round
Chose two rage feats changing your form and costing 1 action. Both activate at the same time.
Level 12 feat: [3 actions] shoggoth wrath
[rage, blah, blah]
Frequency: 1/rage
Make a Strike with every attack you have. You don't suffer MAP.
Optional rule
The duration of your rage is 1d6 rounds instead of the normal Duration. The duration of fatigue at rage end is 1d3-1 round instead of the normal duration. Even if you roll 0 rounds of fatigue, the rage stops nonetheless - although you can immediately spend an action to activate a new rage.